I have the NADA RV Appraisal Guide for Jan - Apr 2011...yikes it is almost two years old. This older guide has the Trade-in for the various 21 foot models as M-210 LTD $5,560, M-211 as $5,830, M-212 as $5,290. I checked the 2006 models - seems depreciation is around $500 per year. Retail shows about $1,100 more than trade-in.
With the leaks subtract some, with lite use add some...
My best guess is $4,500 to $5,000.