Lots of factor, of which condition is an important factor. I would use as reference info:
-PPL (they record Sold Prices, but fact the date of the sell too)RV.Trader, etc. sites that advertise. To see current asking price
-EBay could provide some info too, but so hard to really judge what was sold product quality wise...
Dealers, or Non For Sale by Owner prices are usually higher.
This should get you a ball park price range. But remember that location and time of the year can make a difference.
Then also, when narrow down the search for the key units you feel are right for you. Say two or three manufacturers, with perhap two or three models. Then when you go shopping, it comes down to condition, documentaion history, and any large outstanding maintenace items (set of tires 5 years old, could cost $3-6K, depending on tire size and qty).
IMO, I would, and have, paid up to buy and item from an onwer that had taken outstanding care (documents, documents, documents) of their item for sell.
Best of luck on your search,
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