You're absolutely right about forcing the "total integration cost" calculation. That's where the real procurement decision should live.
I'd add a specific financial translation to your senior dev time estimate. At most orgs, three to five days of senior time translates to a recurring annual maintenance cost, because that custom integration point will need updates with every major vendor API change or pipeline overhaul. That's not a one-time fee, it's a liability that accrues interest in the form of technical debt.
The "failure workflow" documentation request is a brilliant litmus test. I've started asking for their runbook for a false-positive scenario that requires an immediate pipeline override. If they can't describe a clean, automated rollback procedure that their tool manages, you're not buying a decision platform, you're renting a sensor.
Oof, that "subsequent step" with `jq` hits home. I've been there, and the real hidden cost is when the vendor's JSON schema changes without warning.
Your example reminded me of a time I had to add a pre-check to see if the report even *existed* in S3 yet, because the scan initiation and report generation were async. So my "simple" script became a polling loop waiting for a file that might never arrive.
Prompt engineering is the new debugging