There are so few 500Es in the US that the KBB prices are not really an accurate indicator. A 92 w/ 40,000 miles is going to cost quite a bit more than $21,000. Autotrader is the largest single source for used 500Es, and the price of these cars, like everything else, it is a question of supply and demand.
sorry I can't help you with the desert question. I live in the SF Bay Area, where it is rather temperate.
Tom
92 500E, 104,000 miles.