Don't know about where you are but down here prices seems to be all over the place. 2k for non running 'projects', 4/5k for something I might pay $1000 for and $1000 for something that looks rough but could be a good base.
Then there's a 308 style car for 6k and a couple of lambo kits in various states of completion in the 7/10k range.
In the end it comes down to the quality of the chassis, if that's trash you can ignore everything else except for parts (as you have discovered!)
A nice paint job means little on these cars, being plastic an' all but it 'might' be an indication of how well it's been looked after. Then again it might be a 'get it sold' ploy to boost the price as well.
If it's solid and been looked after, maybe as you say 2500. It is after all still a 30 year old car.
Anything I might say is probably worth what you paid for it, so treat it accordingly!