The problem, for lack of a better way to put it, with even near perfect examples of the AMX is that there aren't enough people who love them for them to be as valuable as you might think. It all comes down to market demand with collector cars - then add in provenance such as race history, etc plus accuracy of restoration, or level or originality - to see prices really go up.

If you got your hands on an original car that raced in the Trans-Am series you've got some serious cash, but probably still not as much as a TA Challenger, or a Boss 302 Mustang or a Camaro that raced in the same series. More people just happen to like those cars - that's not a bash against the AMX, it's simply a fact. Neither right nor wrong, just is. But the upside to that is that you can pick up a truly top line American Muscle car for relative peanuts compared to things like Olds 442s, Yenko Camaros, Hemi 'Cudas, etc. Certainly the lovers of the AMX don't love it any less than lovers of other muscle cars (or antiques, Euro sports cars, etc) do, there simply aren't as many of them. Supply and demand, really.

And after all that, $15k sounds a bit cheap for even just a "really nice" example.