It would be a very bad idea to enforce the contract in the variant of Kastoroff just described.
Enforcement would be bad for Kastoroff: he would have to perform for $10,000 less than he was willing to.
It would be bad for the school district as a contractor in financially disadvantageous contract is likely to cut corners and do poor work.
So why enforce the contract when not enforcing it leaves the school district no worse off than if the mistake had not been made?
This is a good example of unconscionable enforcement.