I think stage1gn's post is dead on, although I'm not surprised to see it poorly received in this forum. GNX's aren't fetching what they did a few years years ago, most likely for the very reasons he identified. Should GNX prices have risen into the six figure range when they did? Maybe not. Will they return there one day? Probably. But they currently lack the one thing a highly collectible car needs...exclusivity. "What?!?" the purists will cry. With only 547 built, they are the most exclusive American iron of their era. But that's not the problem. The problem is, at any given time, any collector worth his salt can find a half dozen GNXs worth considering in less than a day. Until these cars find their way back into the hands of the true collectors with deep pockets, and aren't constantly offered for sale, they aren't going back to the price range previously attained. Just my 2 cents. Again, unlikely to be too well received here. 
