Build it with the customer's own numbers, not your ROI calculator. Start from a metric they already report, show the current cost with the assumption stated plainly, then model the change conservatively and label it as an estimate. Include implementation cost, internal time, and payback period. A CFO trusts a modest case with visible assumptions far more than a spectacular one they cannot audit.
Why interviewers ask this
Budget scrutiny is high and most deals now need a finance sign-off, so a rep who can only sell to the user is limited. Interviewers want to know whether you can speak in payback, risk, and cash rather than features, and whether you build the case with the customer so they own it. They are also checking intellectual honesty, since inflated numbers get torn apart in one meeting and take the deal with them.
How to structure your answer
- Use metrics the customer already tracks and can defend.
- State every assumption on the slide, in their words.
- Model conservatively and include implementation and internal time.
- Lead with payback period and risk, not total savings.
- Have the champion co-author it so it is their case, not yours.
Example answer
The single biggest thing is that I do not build it, we build it. If the numbers are mine, finance treats them as marketing. So I sit with the champion and we use figures they already report internally. Then I go conservative on purpose. If they tell me the process takes six hours a week, I model four, and I say out loud that I have cut it back, because the moment a CFO finds one inflated assumption they stop reading. I always include the unglamorous side: implementation, the internal hours their team will spend during onboarding, and what happens in the first quarter before anyone sees a benefit. I lead with payback period, not total savings, because a five-year savings number means nothing to someone managing this year's plan. On one deal the entire case came down to a single slide with three lines: what the current workaround costs annually, what it costs to fix, and the month they break even. The CFO asked two questions about the assumptions and signed. Big decks lose. Auditable arithmetic wins.
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See how it worksFollow-up questions to expect
- What do you do when the customer will not share their numbers?
- How do you handle a CFO who challenges your assumptions on the call?
- How do you build a case when the benefit is risk reduction, not savings?
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