You need to remember condition makes a massive difference in price, yeah you can find a 2+2 for 12k but you will need to fix a lot of rust plus interior so technically you're in it at 20k.
Recently a 260 2 seater sold for 18.5k with the seats with some tears and about 5 'visible' rust spots in the paint.
I'm keeping an eye on the 240z for sale online.