Fixed fee or cost-plus: how should you pay an event production agency?

You're looking at two production proposals for the same paid conference. One agency offers a fixed fee. Another says it works on cost-plus. The second number is lower, but you're still not sure what happens when the venue changes its access hours or your programme needs another room. Which bill are you actually agreeing to pay?

For a well-defined event, start by asking for a fixed price for the agency's own work, with supplier costs and changes explained separately. Where important work is still uncertain, buy a limited planning stage first or agree a carefully controlled arrangement that pays actual costs. Choose the model after you've established what can change, who bears that cost and how you approve more spending.

A fixed fee can be a sensible purchase. It can also cover only management while leaving most of the event budget open. Cost-plus can make supplier spending easier to inspect, but the words alone don't tell you whether the agency earns a fixed amount or more money whenever you spend more. You need the calculation behind the label.

This guide explains those differences, works through the same fictional event under several pricing arrangements, and gives you a payment brief to use with agencies. The aim is to choose a deal you can afford and manage while still paying the people delivering your event fairly.

First, find out which part of the price is fixed

Before comparing proposals, ask the agency to split its own services from purchases made for the event. Producing a running order, coordinating technical suppliers and supervising the build are services. Hiring sound equipment, ordering signage and booking freight create other costs. A proposal can fix the first group while estimating the second.

That distinction matters because the agency's fee may be a small part of the total you authorize. If management is fixed but supplier costs are reimbursed, a higher equipment invoice can still increase your bill. You haven't necessarily bought a fixed-price production package, even if the proposal's cover says "fixed fee".

Ask a direct question. "If you deliver the agreed event without any change from us, can the total payable increase? If it can, which lines can change, how are they calculated and what approval do you need?" A useful answer names the exceptions. An answer about flexibility needs another question.

You also need to know who contracts with suppliers. Your business might book the venue directly while the agency buys technical services. Alternatively, the agency might contract and invoice for the whole production. Record who owes each payment and which terms apply if the event changes. The pricing model doesn't answer that by itself.

Our guide to an event planning scope of work helps you describe the services you want priced. For this decision, take that agreed scope and separate the agency's service fee, supplier purchases, reimbursable expenses and optional additions. Then you can see what each proposal commits you to.

Don't ask an agency to guarantee a programme your own team hasn't decided. Explain what remains open and request a price for resolving it. Paying for a useful planning assignment can be better than pretending an early estimate is a final quote, then arguing about every decision that follows.

When a fixed price is the better starting point

A fixed price works best when the agency can understand and price the required result. You know the rooms, delivery dates, operating hours, key equipment requirements and your team's inputs. There may still be ordinary planning decisions, but the agency isn't being asked to price several substantially different events at once.

The US Federal Acquisition Regulation describes a firm-fixed-price contract as placing cost responsibility on the contractor for the agreed work. That definition belongs to US federal purchasing. It usefully illustrates the distinction between a price that stays agreed and one recalculated from actual costs, but your private event contract must state its own terms.

For example, an agency can price the management of a repeat conference against a confirmed plan. If its producer takes longer than expected to finish the agreed running order, that doesn't automatically become extra billable time under a genuinely fixed service price. Your agreement should distinguish completing the original job from doing work you add later.

The agency also keeps the benefit if it delivers efficiently, subject to the contract. That's part of the bargain. You shouldn't demand an automatic refund because it completed the agreed work with fewer hours than it expected while also expecting it to absorb every underestimate. Judge the delivered service against what you bought.

The tradeoff is that a firm commitment may cost more than an optimistic estimate. The agency has to allow for uncertainty it accepts. Decide whether the additional price is worth the reduced exposure. You need to compare a credible expected bill and the consequences of changes, rather than treating every allowance for uncertainty as unnecessary margin.

Read the exceptions carefully. A fixed production price might exclude additional rooms, delayed access, changed quantities or a specified increase in supplier charges. Those exceptions may be reasonable. They also determine how much certainty you've actually purchased. Ask the agency to explain how each exception would work at your event.

A fixed price is a poor choice when it pressures either party to hide missing work. If rehearsal, competent supervision or required accessibility support is necessary, include it in the requirement. A low price for an inadequate service doesn't become good value because it cannot increase.

Use hourly or daily billing for work you can limit and review

Time-and-materials means paying agreed rates for time worked, together with the materials or expenses allowed by the agreement. In event production, you might use it for an early feasibility assignment, a specialist's site investigation or additional planning work whose extent is genuinely difficult to estimate.

The US federal description of time-and-materials distinguishes specified hourly rates from actual material costs and requires oversight and a ceiling in that purchasing context. For a private event, take the practical lesson and negotiate your own limits. Agree the rates, evidence, reporting and spending authority before work starts.

An hourly rate tells you the price of time. It doesn't tell you how much time the assignment needs. Ask which role will do the work, the estimated hours by task and the result expected at the next review. A senior producer's higher rate may still produce the better purchase if they finish the right assignment with less supervision.

Define a working day too. Does the day rate cover a stated number of hours? When does overtime begin? How are travel, waiting at the venue and work across midnight treated? What happens if your meeting is cancelled after the person has reserved the day? These questions are especially useful when comparing event-day and planning rates.

For a separate fictional hourly assignment, suppose you approve 20 hours at $100 an hour, with no other charges. The authorized amount is $2,000. At 15 hours, the producer reports that completing the required work will take another 10 hours. That means a proposed total of $2,500, not $2,000. You can approve the extra $500, reduce the remaining assignment or agree another response before more work proceeds. The example assumes the contract requires approval to exceed the authorized amount. An estimate alone wouldn't establish that limit.

Put the first approval point before the spending limit. If the agency reports only after it has used every authorized hour, your next choice may be to pay more or stop with an incomplete job. Ask for an early warning showing work completed, hours used and the estimated effort still required.

For an uncertain venue assessment, you could authorize an initial block of work that ends with a recommended production approach and a priced delivery proposal. Agree what happens if the investigation reveals a bigger problem. The agency should return with options before undertaking the next stage, rather than treating the first instruction as permission for unlimited development.

Avoid making your own indecision invisible. Repeated programme changes and separate feedback from several managers consume real time. Consolidate instructions and keep an agreed decision record. Otherwise you may blame the charging model for a bill that grew because your team repeatedly reopened approved work.

Cost-plus needs a complete explanation of the plus

Under a cost-reimbursement arrangement, you pay the costs the agreement allows, supported by the agreed evidence. The agency receives payment for its own work in addition. That payment might be a fixed amount, a percentage of specified costs or a combination. Ask which one is being offered before judging the price.

The US federal definition of cost-plus-fixed-fee separates reimbursed costs from a negotiated fee that doesn't vary simply because actual costs change. It also allows fee adjustments for changes in the work under its rules. Again, this is a defined federal contract type, rather than a rule automatically governing your event. Your proposal should explain the actual calculation just as clearly.

For an event, a fixed management fee with reimbursed supplier purchases can make sense when the management assignment is known but some purchasing decisions remain open. The agency receives the agreed fee for that assignment. You pay the approved purchases, which can move as you make decisions or confirmed prices replace estimates.

A percentage arrangement behaves differently. If the fee is a percentage of eligible supplier spending, a higher eligible cost increases both the purchase cost and the fee. That may be an acceptable way to pay for purchasing and coordination. You still need to understand the incentive and the amount your business will owe.

Write down the eligible spending. Does the percentage apply to technical suppliers only, or also the venue, catering, travel and taxes? Does it apply to purchases your team makes directly? Is it calculated before discounts and credits? Can another handling charge be added to the same invoice? Each answer changes the comparison.

Also ask whether a markup is applied to cost or whether the agency is quoting a margin on its selling price. Those calculations differ. Request a worked invoice showing supplier cost, each charge, taxes and the final amount payable. You can then compare the actual bill without having to guess what a percentage means.

Here's a small, separate illustration of markup and margin. Start with an invented supplier cost of $1,000. A 20% markup on that cost adds $200, making the selling price $1,200. A price that gives a 20% gross margin is $1,250, because the $250 difference is 20% of the selling price. These percentages are examples, not suggested agency rates. Neither calculation tells you the agency's final profit after its other costs.

You don't need to negotiate using those labels if they make the discussion less clear. Ask for the cost and the final charge in money, then confirm which calculation will apply to the next invoice. The point is that two proposals using the same percentage can still produce different bills.

Visibility requires agreed records. "Open book" should identify the quotes, orders, invoices and credits you can inspect, and when. It doesn't automatically give you access to the agency's entire business accounts. Agree evidence for your assignment and a practical process for checking it before a payment becomes due.

You can price different stages differently

You don't have to choose one charging method for every part of the assignment. The uncertain work may be concentrated near the beginning. Once the agency has checked the venue and developed a workable technical plan, there may be enough information to agree a fixed price for delivery.

For example, you could commission a limited paid feasibility stage before authorizing supplier bookings. Its result might be a confirmed room plan, access assumptions, equipment specification and an identified list of unresolved costs. Agree the price or time limit for that investigation, what you receive and the decision that ends the stage.

Then pause for a real approval. You may choose the proposed production, request a simpler version or decide the venue cannot support an affordable event. The first appointment should explain whether later delivery is optional and what happens to the planning files if another provider takes over. Paying for planning shouldn't quietly become permission to book the production.

When you compare later delivery proposals, handle that planning payment consistently. If every option follows the same completed investigation, show the money already spent separately from the remaining choice. If an agency credits that payment against its delivery fee, record the actual credit. Don't add the original planning charge again to a total that already includes it.

A mixed arrangement also needs clear boundaries. Suppose the lead producer's management is fixed but specialist design is hourly. State which work belongs to each charge and who approves moving between them. Otherwise the same revision can be described as included production work in one conversation and additional design time in the next.

Choose stages because they resolve a real uncertainty. Splitting a straightforward repeat event into many billable approvals can create work for both sides without improving the decision. The useful question is whether the next stage will establish information you need before making a larger commitment.

Compare the same conference under three proposals

Let's use a fictional paid conference run by Mill Lane Events. The names, prices and changes in this example are invented to explain the calculations. They are not market rates, customer results or a forecast for your event. All amounts here and in the separate examples are USD, with applicable taxes and mandatory charges included consistently. There are no other charges in the example.

Mill Lane has confirmed the main room, programme, access hours and production requirements. Three proposals cover the same agency services and supplier specification. The event's other expenses, ticket revenue and internal staffing are unchanged and excluded from this production comparison. Nobody is promising higher sales under one pricing arrangement.

The first proposal is a genuinely fixed production package of $42,000. For the unchanged scope, the agency accepts increases or decreases in its own delivery costs. Assume the reviewed contract has no price-adjustment exception affecting the scenarios below. Changes requested by Mill Lane need a separate agreement.

The second proposal reimburses approved supplier costs and adds a fixed management fee of $6,000. Supplier costs are initially estimated at $34,000, giving an expected total of $40,000. The management fee covers the specified services even if the unchanged assignment takes longer than the agency expected.

The third proposal reimburses the same eligible supplier costs and adds a 15% management charge. At $34,000 of supplier spending, the charge is $5,100 and the expected total is $39,100. For this fictional proposal, that percentage is the entire agency service charge and includes the same management services. No other markup applies.

At the opening estimate, the fixed package costs $2,000 more than the fixed-management-fee arrangement and $2,900 more than the percentage arrangement. Those differences buy a different allocation of cost risk. They don't establish which agency is better or which proposal will finish cheapest.

Now suppose approved supplier purchases for the unchanged specification cost $40,000. The fixed package remains $42,000 under the assumptions stated. The fixed-management-fee arrangement becomes $46,000. The percentage arrangement also becomes $46,000 because 15% of $40,000 is $6,000. Mill Lane needs to know whether it can afford those variable outcomes before choosing.

If the same specification instead costs $30,000, the package still costs $42,000. The fixed-management-fee arrangement totals $36,000, while the percentage arrangement totals $34,500. Here the buyer benefits from lower actual costs under the reimbursed-cost proposals. The agency keeps the benefit under the genuinely fixed package.

These are alternative scenarios, not amounts to add together. They isolate supplier price movement while keeping the required work constant. A different specification, extra room or reduced service could change the fixed package too. Ask each agency to price those changes separately instead of assuming the model protects you from every event decision.

Test what happens when you add work

Cost movement and scope changes need different conversations. In the first case, you still want the same result and a cost assumption changes. In the second, you're asking for a different result. A pricing model that handles one well may still need a clear method for the other.

Suppose Mill Lane chooses the fixed $6,000 management fee with approved supplier costs. It then wants a second session room. That creates additional technical purchases and genuine coordination work. The original fixed management fee doesn't mean the agency has agreed to manage any number of rooms for the same money.

The agency proposes $3,000 of extra supplier purchases and a $750 fixed addition for the new management work. Mill Lane checks the specification, staffing and schedule, then approves both through the agreed process. The expected production total becomes $43,750, made up of $37,000 supplier costs and $6,750 management fees.

Later, a supplier issues a $1,000 credit against an eligible purchase. Under Mill Lane's agreed actual-cost terms, that reduces supplier spending to $36,000. Management remains $6,750, so the revised total is $42,750. The credit reduces the bill once. It doesn't also reduce a fixed management fee unless the parties agreed that calculation.

Under a percentage model, ask whether the existing rate covers managing the added room. Don't assume it does merely because the supplier budget increases. Equally, question an extra management fee if the agency says its percentage already covers that work. The proposal needs to identify the work each charge pays for.

Ask about reductions as well as additions. Removing a room before bookings may save more than removing it after crew and equipment have been committed. A fixed price doesn't automatically decrease in proportion to the work removed. Request the available credit, unavoidable commitments and any effect on the remaining delivery before deciding.

Use a change record that shows the original requirement, requested change, supplier difference, agency fee difference and revised total. Keep the approval attached to the relevant quote version. Your finance lead should be able to follow the calculation without reconstructing a conversation from separate messages.

A spending cap needs an action when you approach it

An estimate, a budget and a contractual cap do different jobs. An estimate predicts the bill. A budget is what your business has allocated. A cap limits an agreed category of authorized spending under stated terms. Writing the same number in all three places doesn't make them interchangeable.

If you agree a cap on agency hours, establish whether it includes all roles and expenses. If it covers total production, identify supplier purchases, taxes and approved changes within it. Ask what happens when the remaining authorized amount won't buy the agreed result. You need a decision before suppliers expect an instruction.

A workable response may be to remove an optional element, choose an acceptable substitute, authorize an increase or stop an uncommitted stage. Agree who can make that decision and when the agency must flag it. A cap without that process can leave both sides arguing while the delivery deadline approaches.

Don't describe an amount as a guaranteed maximum unless the agreement actually supports that promise. Exceptions for every uncertain input may leave you with little practical protection. Ask for a scenario showing the proposed maximum, the exceptions and which party pays if the ordinary estimate proves wrong.

Keep your reserve separate from permission to spend. Your event contingency budget may fund specific risks, but the agency shouldn't assume that seeing the reserve authorizes purchases against it. Name the person who releases it and the information they need to decide.

Reporting should show the latest expected final cost, including work you haven't yet paid for. A low invoice total can be misleading when major orders are already committed. Review paid amounts, unpaid commitments and forecast remaining work together. That gives you a chance to change an optional purchase while it can still be changed.

Agree the evidence before the first supplier booking

With reimbursed costs, check what you will receive and how it relates to your event. A supplier quote supports a proposed purchase. An accepted order shows a commitment. An invoice states an amount requested. A credit changes that amount. One document doesn't necessarily replace the others.

Ask for a cost record that connects the purchase to your approval and the supplier's actual charge. If several events share a delivery or crew booking, agree how your share is calculated. Otherwise a broad supplier invoice can be genuine while still leaving you unable to establish how much belongs to your event.

Treat agency-owned equipment explicitly. The agency may legitimately charge hire for equipment it owns. That's a quoted service or hire charge, rather than an outside supplier invoice to reimburse at cost. Decide how it's priced and shown in the proposal so ownership doesn't become a surprise during reconciliation.

Rebates, supplier commissions and credits also need agreed treatment. Ask which ones exist, what the agency retains and what reduces your bill. That disclosure affects the cost you compare. It doesn't mean every agency must use the same business model or that every retained benefit is automatically improper.

Avoid building the relationship around repeated demands for documents nobody agreed to provide. If you want actual-cost evidence, make it part of the purchasing arrangement before booking. If you're buying a fixed package, decide what evidence you need to accept delivery rather than assuming you bought an audit of the agency's profit.

For a recurring series, keep the pricing assumptions attached to each date. A cheaper repeat date might reuse planning or equipment, while a different venue introduces new work. Compare the actual scope and purchases at each event. A series label doesn't establish that every date has the same cost or requires the same effort.

Match payment dates to work and commitments

A fixed fee doesn't mean a single payment after the event. The agency may need staged payments for its work and funds before committing supplier deposits. Agree the schedule alongside the price, because a deal you can afford overall can still require cash earlier than your business has available.

Australia's business.gov.au guidance on preparing a contractor agreement recommends stating the fee calculation, tax treatment, payment timing and the work that triggers progress payments. Use those practical questions for your agency agreement. Payment rights and remedies still depend on the terms and law applying to your business.

For each payment, identify the amount or calculation, due date, required evidence and recipient. A management milestone might follow delivery of an agreed plan. A supplier deposit relates to a particular booking. Describe both clearly so you don't pay the same deposit directly and again through an agency reimbursement.

Ask what your payment commits you to. Can the supplier booking be cancelled? What amount is recoverable, by whom and under which deadline? What happens to unused purchasing funds? A payment schedule should explain those practical consequences, rather than leaving every advance under a general heading of "deposit".

Check the position if the agency or a supplier cannot deliver. Who holds the booking? Can you obtain the documents and continue with another provider? What information and rights would you need? Have significant advance-payment arrangements reviewed before commitment. A familiar pricing label doesn't establish protection for the money.

Keep supplier cash requirements separate from assumed ticket receipts. Sales may arrive later than the deposit deadline, and money showing as sales isn't necessarily available cash. Plan against confirmed payment obligations and your actual funding position. Don't authorize a larger production purchase solely because you're hoping the next ticket release will sell quickly.

If an invoice is disputed, identify the disputed item and follow the agreed process promptly. Resolve an unsupported charge without casually withholding unrelated payments that are properly due. You want delivery to continue under clear terms while the specific disagreement is addressed.

Check one complete invoice before approving the format

Ask the agency to demonstrate an invoice using a small set of fictional entries before actual billing begins. Include an ordinary supplier purchase, a deposit already paid and a credit. You want to see how the invoice prevents those amounts being charged twice and how it distinguishes management fees from purchasing funds.

Confirm how estimates become actual charges. If you advanced money against a supplier estimate, the final reconciliation needs to show the amount actually due and what happens to the difference. If a credit is expected but not yet issued, keep it visible as outstanding. Don't silently deduct an uncertain amount from a different payment and assume the matter is closed.

Name the people who answer questions too. Your finance colleague needs a contact who can explain the calculation, while the producer needs a route for resolving a disputed purchase approval. Agree how quickly queries are raised and answered around payment dates. That preparation makes it easier to pay correct invoices promptly and investigate the particular item that needs attention.

Keep agency charges connected to work someone actually does

Before agreeing a management fee, check which tasks your team retains and which tools already support them. A proposal can include useful setup or supervision. It can also describe the same activity twice under different service labels, especially when several people contribute to ticketing and arrival operations.

For example, Loopyah's event team access lets you add admins and door staff without sharing your login, and give door staff check-in access to scan QR tickets on their own phones. If the agency is coordinating the door, price its actual preparation, staffing and supervision around that capability. Account access doesn't perform those human jobs for you.

Ask the agency to explain any separate equipment, software or administration charge in that context. It may cover a genuine requirement, such as a different admission setup or extra support. You need the reason and the required result before deciding whether to buy it. Don't assume every additional tool is necessary just because it appears in the proposal.

The same applies to reporting. Define whether you need a supplier cost reconciliation, an operational debrief or analysis that helps make a commercial decision. Those are different assignments. A percentage management charge doesn't tell you what report will arrive or whether someone will explain it to your team.

Use your agency and internal team responsibilities to identify retained work before comparing totals. If one proposal expects your operations manager to supervise the build and another supplies a producer, the services differ. Record your team's required time separately instead of treating it as free or automatically turning it into cash savings.

Don't turn a fee negotiation into a demand for unsupported results. An agency controlling technical delivery may have little authority over your ticket price, programme or marketing. If you discuss a performance payment, use an outcome it can influence, reliable evidence and a clear calculation. Avoid paying twice for ordinary delivery already required by the base fee.

Use this payment brief with shortlisted agencies

You can send the following questions with your agreed service scope. Ask each agency to answer against the same event version, rather than rewriting your requirement around its preferred charging model. This is a buying brief to discuss and have incorporated into an appropriate agreement, not a complete legal contract.

• Identify the event, dates, rooms, operating hours and scope version used to price the assignment. State unresolved decisions and the assumption used for each.

• Separate agency services, third-party purchases, agency-owned equipment, expenses and optional work. Show the complete amount payable, with consistent tax treatment.

• State which prices are fixed, which are estimates and which are subject to an agreed limit. Explain every adjustment mechanism and exception.

• For time-based services, list roles, rates, estimated hours, the length of a day and the treatment of overtime, travel and cancelled bookings.

• For reimbursed costs, define eligible purchases and the evidence provided. Explain discounts, credits, commissions, rebates and any fee applied to them.

• For percentage charges, define the exact cost base and calculation. Show a sample invoice and explain whether other management or handling fees apply.

• Name who contracts with each supplier, who can authorize a booking and who pays. State how advances and supplier deposits are recorded and reconciled.

• Describe the report showing paid costs, unpaid commitments and the expected final bill. Agree when it arrives and when likely overruns must be raised.

• Explain how additions, reductions and substitutions are priced. Identify the people authorized to approve them and the required written record.

• State the payment milestones, evidence, due dates and end-of-job reconciliation. Explain unused funds, outstanding credits and the terms applying to cancellation or early termination.

Ask the agency to walk you through one ordinary change using those answers. A second room, extended access period or removed rehearsal often reveals more than a general assurance that the fee is transparent. You should be able to follow the resulting invoice before signing anything.

Pay attention to what the agency needs from you too. It may need confirmed quantities, consolidated feedback or a decision by a supplier's booking deadline. Include those obligations in your planning. You can't ask the agency to accept a fixed result while reserving unlimited freedom to change the inputs without consequence.

Record why the chosen arrangement fits your event

Here's how Mill Lane's decision could read after clarification. The choice is illustrative. It isn't a recommendation that every conference should reimburse costs or pay the same fee. Another event with less management time available might reasonably pay more for a genuinely fixed package.

Mill Lane selects the fixed $6,000 management fee with approved supplier purchases because its delivery scope is sufficiently clear and its operations lead can review purchasing. The opening supplier estimate is $34,000, so the expected production bill is $40,000. The business accepts that actual approved supplier costs can change that total.

The event director, Nia, approves purchases and scope changes. The operations lead checks proposed purchases against the specification and maintains the cost record. The agency producer coordinates suppliers after approval. Nia names a backup who can make decisions while she's unavailable. Neither a budget entry nor a supplier recommendation is permission to place an order.

The decision record states that agency management of the original scope remains fixed. New rooms or event dates require a priced change. Supplier costs are reimbursed only within the agreed purchasing terms, with invoices and credits available for review. Any proposed replacement affecting the audience experience requires the appropriate internal decision as well as a price check.

Before appointment, Mill Lane also tests the $40,000 supplier-cost scenario from the comparison. It understands that its resulting production bill would be $46,000. If the business couldn't fund that possibility, it would need a different commitment, a reduced specification or stronger cost limits before proceeding. The lower opening estimate alone wouldn't justify the appointment.

New Zealand Government Procurement's contract administration guidance connects checking charges and approvals with recording agreed variations. Although written for public procurement, that discipline is useful here. Mill Lane keeps each commercial decision attached to the current scope and cost record so the final invoice can be checked against actual approvals.

Review the arrangement while you can still change the plan

Once the agency starts, review the fee calculation alongside delivery. Are the agreed people doing the work? Are supplier commitments appearing before payment deadlines? Is the expected final cost changing for a reason you understand? A clear agreement only helps if someone uses it during planning.

If the management fee keeps increasing, look at the cause. Your programme may be expanding, your team may be issuing conflicting instructions or the original scope may have omitted essential work. Those problems require different decisions. Use a specific change and its cost to resolve the issue, rather than simply demanding that the agency "stay within budget".

If the purchasing total falls, confirm what that means under the chosen model. Reimbursed costs should follow the agreed actual-cost calculation. A genuinely fixed package follows its own terms. Don't switch between those expectations depending on which produces the lower bill after the work has been done.

Your actual-versus-forecast event budget should show the latest expected bill alongside other event spending. Keep approved additions, likely unresolved costs and available funds visible before another commitment. An agency can stay within its own service fee while the overall event becomes unaffordable elsewhere.

For your next agency discussion, bring the same confirmed scope to every shortlisted supplier and ask for the total under an unchanged event, a cost increase and a specific additional request. Choose the arrangement whose calculations you understand, whose risks you can carry and whose approval work your team can actually perform. Have those answers written into the agreement before the first booking.