Builder is going to build a building for Owner. It will cost Builder $80,000 to do the job. After Builder has completed the building, and before Owner has paid anything (that's not likely to happen in real life but it simplifies our problem) Owner breaches.

Builder's "loss in value" is of course the $100,000 she would have received. If we give Builder the $80,000 as an "other loss," she will get a total of $180,000. If the contract had been performed, she would have received only $100,000. So we can't give her that extra $80,000 because it would put her in a better position than performance would have.

And remember: the goal of contract damages is to put the non-breaching party in the same position that performance would have--no better, no worse.