Welcome to the forum bakercabin,
Finding an average price for any used RV will take some homework on your part. Prices can vary from one U.S. location to the next, but...
Once you've decided on a model#/floorplan, you can use the NADA value as a starting point. It's been said that the NADA 'high retail' figure is a bit too high in the current economy, but maybe someone else can offer more input on that topic.
You can also search online (using Google, for example) for specific models & get some prices to compare. There are also several sites that list used RV's for sale. The down-side to researching online is that many times the seller isn't familiar with the brand and/or the options/upgrades and may not specify those items. You'd need to be fairly familiar with a specific brand/model to know whether or not you're comparing identical units.
The floorplans & specs are available on the Keystone website - scroll to the bottom of the page, see "Previous Year Specifications":
http://keystone-montana.com/index.php?page=specs
http://keystone-bigsky.com/index.php?page=specs
There was a Dometic refrigerator recall (not specific to Keystone) which included the 2004-2007 Montana & Big Sky, but (logically) that should have been repaired before now.
http://www.keystonerv.com/media2/pdf...02004_2007.pdf
Hope this helps!
Terri, the Chevy co-pilot