+1 on 20k plus... on the upside its not overly molested, just neglected. Rust looks to be potentially significant, torque box looks shot, door and rear 1/4 show some rot... trunk looks suspect.
Engine bay looks pretty stock and the interior needs a gutting and replacement, but thats easy.
Body and paint could go 10k easy, depending on what you can do yourself vs pay for metal replacement. Interior 1-2k, basic engine/trans rebuild 4-6k, brakes and suspension another 1k, new bumpers and chrome or replace window frames another 1k, all the other "minor" stuff (trims, badges, etc) and you get to 20k without trying too hard.... so if you paid less than 10k you would only be slightly upside down as nice C codes seem to be in the mid 20's recently...
IMHO if its running and driveable 5-7k would be a "reasonable" price to both sides. If someone really wanted one 8k would be pushing its value. On the plus side it does seem to have have the OEM fiberglass. Have seen far worse cars ressurected, and it is a GTCS/HCS....
Odd it has a 67 gas cap and front seats....