Tough question. I have seen very nice 90's+ 300-zx's sell in the $8,000 range. The high for a factory authorized 240-z restoration is about $34,000.
An insurance company would value a 280 at about $3000.
based on the market, I would set $8,000 as a "floor", if you will. A true z affectionado, looking for the "right" 280, will pay a pretty penny for one like yours. The average person, with, say $10k, would likely opt for a newer z if they werent "280-Z" nuts. IF you are selling it, hold out for the right buyer who truly appreciates/wants a mint 280. That buyer would pony up, say, $10-$15K based on their love of the 280.
My 240 is "worth" $2,000. But I have put $8,000 total into it, because, that's what it is worth "to me".
Over $15k, I think you find few takers, only because you are getting into the range where a $2k 280 can look/run awful good with a $13K investment, $3000 paint job and $10,000 in mechanical work. range your 280 at $8-15K, but look for a slow market.
The problem is that for $8000, I can buy an all original 240Z. It won't be mint, but it will be in very good condition. That is what you are going to up against. The 280Z just isn't that collectable. However, hold out for the right buyer.