That sounds high to me, especially since it's a 91. What about it does he say makes it a collector's edition? Just being a '91 (even a '91 Twin Turbo) doesn't do so, save for the fact that the 90-96 300ZX's were the last made (prior to the release of the 350Z in 2003). In any case, the condition would have to be beyond perfect for it to be worth anything close to that. My advise: if you want one like it, check on E-bay. You should be able to find one for much cheaper that's in equal condition. In any case, take one thing to heart: the condition in which the car is kept is far more important than the mileage.
I hope that helped some.
- Jeffrey
P.S. I just got on Autotrader and did a search for a '90 - '92 300ZX for under $12k. Got one with only 35k miles here in my own town (and OK is a *bad* place for finding cars, generally speaking) so you shouldn't have trouble finding something better. Just be a bit patient when looking for cars and you'll find a great deal.