A lot of it is greed, or people who bought one and paid x, so they feel they should at least get x for it when selling it. Unrealistic hopes, usually, since the radios de-value to about half of retail immediately, then continue to decline as newer models are released and the radio becomes more and more obsolete. Then, finally, after a LOOONG time, the radio begins to pick up in value because all the cheap ones were bought up and resold, so all that are left are the more expensive, unrealistically priced ones. We are at this point for Astro Sabers and XTS3000, they used to be dirt cheap and commonplace, but they have dried up of late. The XTS5k / 2500's are back in the cycle further, at the point where the prices have declined to about half their initial "street" value of somewhere around $1000. Eventually, the radio enters the parts/museum piece stage when the price can go up quite a bit, but typically only for rare or unusual old radios.
As usual, shop the seller among the reputable ones, and look for a radio that has full and legitimate tags. The only other pitfall is getting ALT radios, but most of them in the XTS series have come up for sale cheaply, and have been since sold off so there's not that many of them left out there in general. Features like FPP, etc. used to command a premium of somewhere around $100-$200 or more in the early days of the radio's history, but after all the leaked copies of Depot, etc. out there have become commonplace, any idiot can "pump up" his feature set; this makes the flashcode of the radio pretty much irrelevant as far as price goes now.
Basically, as long as you do your homework, and the radio is legit and looks half decent, finding one for $500-$600 is the idea. You can pay more, but usually that is to sellers that aren't in touch with "street" prices, or they may have a pristine radio they feel is worth more than what others are asking, sometimes with additional accessories, etc.
