Let alone that we basically award this work one estimated number and the ability to get through a paper work hurdle.
If you start making those rules complex it becomes hard to compare bids to each other, since your bids are likely to become non-transitive based on the scenarios.
Cost plus is the normal way this is side stepped, but unless you have a system that is mostly ethical actors that can break down. And if you have all ethical actors pretty much any system will work...