My guess is that the rust may be more extensive than what is obvious. Check the floor pan, the trunk, bottom of the doors, lower front fenders (behind the fender well), core support, especially near the battery and the base of the windshield and rear window . It would be my opinion that 12.5k is a high price considering the cost of panel replacement, body work and paint. I have a very straight, rust free, 79 model and have been quoted between 14k-22k for body work and paint. My car however is not being done with an eye towards ROI. It has been in the family since new, and will continue to stay in the family, so resale value isn't being considered at all in our restoration. Having said all of that, more pictures would certainly help. Have you verified that it is a numbers matching car? Engine numbers, trans numbers, differential, firewall? Those are the main items considered, however it can get much more extensive than that.