Sorry to say that value is a perseption of condition by an interested buyer.Here in Texas its all about how good the a/c works not so much rust unless it was a coastal car.Being from up north Im sure rust is more of a main concern and is probably frowned on with disguist .The motor runs like a top and the trannie shifts great youll be good at $2000. Voluntarily give up the fact of the rust and see if you can get estimates to economically fix the issue and include that in your bargaining ,your showing that it is an issue but thats why the buyer is getting such a great deal.