I would think $15k for a rebuild is a lot. I mean 3k in body, 3k in paint, 3k in interior, 3k in motor what's the other 3k for? I know suspension brakes and tires. And that is good money for that stuff too!
lets see, how complete and nice is a 3K ta currently?? there are additional body parts you may need to replace, trans, exhaust, weatherstrips, wheels and tires, most likely a windshield, wheel flares are always an issue, some grills, decals and stripes......that alone is easily over 3K.
you are assuming you can go out and buy a solid, straight, complete, running, rust free t top trans am for 3k, because if its not, no body shop will touch the paint for 3K, and any performance mods will run the engine well over 3K. any real shop trying to make any profit at all, would have a hard time supplying and building a nice TA for 15K without cutting corners