If the car is in great shape, $50k is not out of the question. I believe there was a Batmobile in Roundel recently for $80k, advertised as immaculate and restored. That price was a bit absurd, but you get the idea. If you can find the right buyer, with the cash, the sky is the limit.
I would like a Batmobile myself, but I won't be in a position to have that kind of a car collection for at least another five years. At least I have my Art Car models to tide me over.