I'm not sure why
someone would pay $70,000 for a GTA. Certainly I wouldn't, even if I had it....but than again, why would someone pay almost that much for a 356 VW (oops, I mean Porsche)?
I think it's whatever the market will bear, and compared to some of it' contemporaries, the GTA has been cheap.
Personally, if I were going to sepnd that kind of money, I would rather have a 1900...and you should be able to buy a really nice 1900 (except a Zagato) for that