$15,000 - $17,000 depending on condition would seem reasonable... but don't expect to see consistant 'market value' pricing like with used cars.
After spending 6 weeks searching for our 06 Eagle Outback we saw some interesting pricing. 14K for a 04 Hawk OB in great nick, 18K for an 97 Swan OB and 23K for an extremely well loved 05 Swan OB.