You could also try calling around a few dealers to get the book wholesale value which they all have - and then calculate the % markup for your area
For example we just got a wholesale value on a 99 34' Excella at $29,900.00 Canadian. Depending on the mark up dealers take should give you a good idea.
You do not necessarily have to check with an Airstream dealer either as most RV dealers have a value book on most makes and models.
Sometime value is personal preference as well and something to consider - opportunity with chance mixed with availability.