Unfortunately, my only thought is this, with my sincerest sympathies - "welcome to the club of defrauded citizens." Many of us are in the club, whether it is stock, real estate - you name it. There is no legal recourse except the endless lawsuits that will occur. Quite frankly, I'm wondering whether it is a matter of time until people take the law into their own hands since the legal redress seems to be far too expensive, time consuming, and unsatisfactory, especially given the huge number of cases to be addressed.
Remember this - who is going to pay more for your land today than it's worth? Nobody. Why should they? Why should your lender buy you out? In fact, they had nothing to do with the issue at all except give you money for the property. It's all about who bears the risk that the builder was a complete scandalous criminal. There are now possibly millions of these people who took the money and are now laughing at the people they took the money from to get it back.
The best I can say and I'm sure it's been tried - figure out who, if any, might have been responsible for making sure everything was as represented to you. It's not a title issue. Was there some type of inspection that was to be performed that should have uncovered this issue? Was that responsibility yours alone? I don't know. I do know this - there are millions of people who were sold poor deals and are now believing that someone has to bear their loss. Unfortunately, like many others, you may now need to sell what you have to pay for this criminal act by a builder and now live more modestly.
Personally, I'm infuriated. As I said, I just wonder how long it might be until this kind of mass fraud rears its ugly head because not enough has or can be done to address the egregious wrongs perpetrated by the likes of your builder.