In Real Estate, I always tell sellers that the best way to price a home is to predict its market value, and list just above that. The best way to predict market value is to look at the sales price of homes like yours, near yours, that have sold in the most recent past. Then adjust up/down for differences in features, quality, and condition. I suspect pricing a portable home-away-from-home is much the same.
In Nov 2015, I drove from Indianapolis to Michigan and paid $6500 for a 1999, 16 ft mint condition Casita
Spirit with double bunks, ice box, and no bath. There is no heater/AC/water heater/awning. It does have a fantasic fan, new fresh water tank and lines, 5 new tires
, all new vinyl blinds. The original upholstery and carpet inside looks perfect. The exterior was just hand washed and buffed.
Hope that helps.