When is it time to throw in the towel?

Hey guys,

I'm in the need of some guidance. I purchase my first Dart on November 1st and I finally got around to getting estimates on the paint and body. Ive got two quotes today one for 15k, the other for 13k. Turns out my ole girl is full of bondo. Those quotes would cover the removal of bondo and paint and what ever else is needed. Both shops have also said there is no guarantee of removing all the rust. I am by no means in a financial spot to pay some one that kind of cash and I am not as gifted as some of the members here when it comes to body work. My question is this, when is too much just too much to continue on with your car?