In addition to the one mentioned above (it was an ok driver, not great), I have seen 3 other GTCs for sale in the last 6 months. They were all quite nice cars but I'm not sure if they are still available:
1) met. grey / red LHD in Peru $40k USD
2) yellow / grey RHD in France 38k Euros
3) blue / grey LHD in UK 30k Pounds
So, I'd say $40k+ is the top. Basket cases (of which there are several around) seem to go for over $10k if the pieces are there.
A fully restored concours GTC sold in Australia a few years ago for $55,000 at auction.
I know it cost the owner more than doubble this to restore.
I guess anything is only worth what someone is willing to pay for it.
The car was in Sydney and now is in Melbourne.