When a vendor relationship goes wrong, the conversation usually starts with the contract value. That is the wrong number to focus on. The contract is the smallest part of what a bad vendor decision actually costs.
What actually gets spent
Onboarding time is the first casualty. Someone on your team spent weeks configuring, integrating, or training on a tool or service that now has to be unwound. That time does not come back, and it was rarely tracked as a cost of the vendor decision in the first place, so it never shows up in any post-mortem.
Then there is the opportunity cost of the alternative you didn't pick. Somewhere in the original search was a vendor who would have actually worked, who is now serving a competitor, or who has since raised prices, or who no longer has capacity. Re-opening a search six months later rarely starts from the same options.
And there is the credibility cost inside your own organisation. Whoever championed the original vendor now has to explain the reversal. That makes the next vendor decision more cautious than it needs to be, which slows the whole procurement function down, not just this one relationship.
Why this keeps happening
Most bad vendor decisions are not the result of bad vendors. They are the result of due diligence that looked thorough but wasn't. A polished proposal and three positive reviews are not the same thing as verified financial stability, real compliance certifications, and a track record with organisations at your actual scale, confirmed by people who have nothing to gain from the introduction.
The fix is not more time spent reading proposals. It's better-sourced options in the first place, evaluated against criteria that would have caught the problem before the contract was signed, not eighteen months into it.