Sorry to hear about your run of bad luck. I have seen many 'falangs' in the same boat.
Your sorta of between a rock and a hard spot right now. Any builder that you can get will want more money than the original cost. They got you by the short hairs.
My advice? Find a home in the area that you like and find out who built it. Tell him the truth and see what you can work out.
I have been lucky in the sense that my wife knows about Thai construction and I know about building in the states.
Good luck guy!