I don’t think you’ll be able to answer this in any clear way. First, a “fair pricing” kind if thing, in whatever market, is normally figured by findings “comparables.” You have a 25yr old car with 15K miles on it. There just won’t be many comparables.
The second part of the ambiguity is that it is going to take a lot of time and money to make the thing road worthy again. It will need tires. The ones on there won’t be of much use (or at the very least won’t be safe). The gas tank may need to be dropped & cleaned out - possibly the fuel lines as well. Maybe not - but that’s another crucial things that needs time and probably money. I wouldn’t get in it without tearing the brakes down & putting them back together. All of the fluids need to come out. The hoses and belts probably need attention …
I just don’t think there is any fair price that you can find beforehand. You’re just going to have to put it out there, get offers, etc. If someone makes you an offer you’re happy with then I’d be inclined to call it fair.
You could glance at the Kelly Blue Book value, but I think you’d probably have to list it in “poor” condition. Even then, KBB will produce a number, but I doubt it will be grounded in anything real.