"DunellenGuy".....When it comes to RV's you can do mathematical equations until you're too old to drive an RV. It's pretty simple to figure out what a selling price should be. Find the year, brand and model you like and then do a search nationwide for a price.
As an example, if you're looking at a 2007 Fleetwood Terra, search everything that is close. Fleetwood made a similar unit called the Fiesta. Any coach within two model years will give you a good base price. Once you know that a 2007 Fleetwood Terra should sell in the $40K - $45K range, you can start narrowing down your search.
Within a few days of searching and looking at adds, you'll know exactly what a clean one and a beat up one is worth.