“Should we build this internally or buy it?”
The question sounds like it’s about money. It almost never is.
What the question is actually asking: how much of your team’s attention do you want to permanently allocate to maintaining this thing, versus running the business it’s supposed to support?
Building looks cheaper on a spreadsheet. The first version usually is. What the spreadsheet doesn’t model is the next two years — the on-call rotation when something breaks at 3am, the engineer who needs to learn an unfamiliar tracker API, the migration when the third-party service changes their pricing model. None of those costs show up in week one.
Buying looks more expensive on a spreadsheet. The first invoice usually is. What it doesn’t price in: the engineering attention you don’t have to spend on the maintenance treadmill. Whatever else that attention could have built.
If we’re talking to a team about a system we’d sell them, and they’re genuinely going to be better off building it themselves, we say so. That’s not generosity. It’s that the worst outcome for everyone is a team that buys infrastructure they should have built, or builds infrastructure they should have bought. Both endings are bad. The question is honestly worth thinking through, not closing.