As Ian says it's mostly down to what someone is willing to pay which is down to how much they want it.
I'm currently going through my guitars to decide which ones I will no longer need as things and needs have changed vastly for me, as well as finding that the profile of any given neck can make a big difference whether or not I can play it or not, as a millimetre in the dimensions of the thickness or neck camber can affect my feel these days and make it difficult to play well.
I'm using eBay and Reverb as a guide to get an idea of what guitars may or may not be worth as well as looking at the adverts on-line of used guitars.
The one thing that really helps in many ways is having the serial number to able be to compare prices and you'll get an idea from the manufacturers about the build of the instrument and in which factory / country etc.
As an example. I have a USA 40th Anniversary Fender Stratocaster (1994) with the 40th Anniversary badge and neck-plate, but in reality it's a USA Standard Stratocaster from a period when they were very good and the ordinary USA Standards are fetching more money in good condition than the Anniversary, because many people don't care for the badges and paraphernalia associated with those models. In some cases a Stratocaster associated with HBM is less desirable than a normal model without the signature and hype.
In truth there is nothing special about those guitars that warrants the extra expense and a lot of people, me included, believe that the only benefactor of peddling these 'special' guitars is gained by the original dealer who probably sold what amounts to a £2000 guitar for £4000. Don't get me started on those JMI amplifiers that were used on the Reunion tour. That was THE biggest rip off of the century. The were bog standard AC-30s that they put a badge on and charged the earth for. How do I know? I got the facts from the man who built them. Steve Giles.
It's a funny old world.