The facts of lost profits can be difficult to provde in many cases, yes. But, with such a well-established appraisal system for real estate and the ability to value the units at exactly what the current developer is now selling them for, I would think the chances would be much better than your average damage award. Making the plaintiff "whole" in this case would be, in effect, giving them what was needed to put them in a similar unit at the same price they'd contracted for. That won't require the same mental gymnastics that your usual contractual agreement for sales of widgets that were never received, licensing revenue on songs never sold, etc.
Funny thing is that I'm reading a case right now in which lost profits were successful and read two yesterday. So, perhaps lost profits are under conventional wisdom considered harder to prove, but I would hardly call them rare. Just one of those things where consulting an attorney is just a good idea for anyone. My point was more the latter and trying to point out how much could be at stake.