They can sell for more than their worth. Don't bother with eBay sales for final bid price on similar units (hard to catch unless you're on it closely). Realistically it depends on condition and what you will have to put into it from your purchase going forward. What is sold as a restored trailer often is prettied up (nice shine job) without underlying values of solid floor or truly
working appliances or fridge being addressed.
RJ Dial's site is based on him being fairly active following price trends. Says it was last updated August 2009 though. Read the red print just over halfway down the page. His site is here >> Price vs. Condition - Airstream Values