Like anything, condition is key, even for 280zx, Z31 & Z32's, there will be those out there for which a 280zx is their favourite car (not many in comparison, but some) I have seen mint 280zx's (although few and far between) attract good prices (for a 280zx )
I also have a 300zx, but a 25th Anniversary Edition, this is a low K excellent condition, 100% original car, I have seen these advertised for high teens to early twenties, and seem to sell for close what is being asked for, condition, rarity & originality are key to good prices, regardless of what it is. (yes, even 280zx's, even ugly babies are loved by the mothers!)
Lee