Some boats are worth less than $0. They will suck you dry (or threaten your life) no matter how low the purchase price. Be careful.
Best advice, so far.
Assume that if you buy a boat for how ever much less than the normal going rate, it will take that much and more (depending on how much you do yourself) to make it right. That can be a good thing too. You will learn a lot about your boat fixing it. OTOH, if you want to sail, and not work on the boat, get one that is in good shape.