The build option is seductive because its cost is invisible in the meeting: engineering time is already paid for, so the build feels free, while your invoice is a number on a page.

It's also partly a loyalty test. Dismissing their engineers insults the room; the CFO is watching whether you can argue economics without arguing competence.

Agree sincerely; the fight you must not pick is ability. Then ask what this would displace on the engineering roadmap. Opportunity cost lands harder when the CFO names it out loud than when you assert it.

The number that decides this isn't the build estimate; it's the standing tax of upkeep, edge cases, and the roadmap nobody budgeted. Name those in their categories, not yours.

Offer the honest split: what they genuinely should build in-house, which is anything core to how they differentiate, versus what's commodity plumbing. Conceding territory you were never going to win buys credibility for the territory you can.

“Engineering says they could do it in a sprint.” They're scoping version one. Ask who owns it once it ships; the room usually goes quiet.

“We already started building something.” Sunk cost is now in the room. Position alongside it, not against it: what's the fastest path to the outcome from here?

“Open source covers this.” Free software, unpriced labor. The same maintenance argument, sharper.