One way to think about prices is to consider that the NADA values are based on the rates of depreciation for CARS. Stickies seem to depreciate at about the same rates.
F/G trailers seem to be more like the rates of depreciation of small airplanes. Instead of taking off "X%" per year of what "this one would have cost when new", you take of "X%" per year of age from its brand new replacement cost.
Since, as a general rule, a new (Casita, Scamp
, etc, etc) will cost more this year than it would have cost last year, this moves the value of a used unit upwards.
For those makes no longer in production, a comparably-sized one can be used.