I've said it before but in my opinion the 280zx was a rapid departure from what made the earlier 240z's so special. A true sports car no frills etc.. The 280zx had all the bells and whistles and tried to move more upmarket in terms of being a luxury GT.
Everything was going computerised and it was more about the gadgets.
I think where the S30z departed the market, the SA22 (FB) RX-7 picked up the torch and ran with it. Early RX-7's are still cheap and I think are under appreciated, as with other early rotary's soaring in value (just look at an R100 now) the early RX-7's (and not many are original anymore) will pick up in value soon.
Check out this R100. (not even Original)
http://www.gumtree.com.au/s-ad/slacks-creek/cars-vans-utes/mazda-r100/1101827150
When was the last time you saw an early RX-7 wearing these awesome wheels? Which remind me a bit of the Kobe Seiko wheels on the S30z.
http://www.gumtree.com.au/s-ad/guildford-west/wheels-tyres-rims/rare-mazda-rims-13-x5-5-110pcd-r100-rx2-rx3-rx4-rx7-series-1/1102215125
I think 280zx values will always remain muted, regardless of how few are left. But happy to be proved wrong and eat my words down the track. I do see a surge in 2 seater values along with the turbo model but I think the 2+2s will remain under loved for some time yet.
If I had the money, space and time I'd get an early series 1 RX-7 now. *Hot tip*.