Depends on whether the site comes with the park model or not.
Offhand I would say the range is $20k-$100k+ with the typical prices in the $38k-$60k arena. That's for the trailer without a site or delivery to a site.
Whether a park model dealer will take a trade-in will depend on what he thinks he can re-sell from his lot - if he even has a lot. Often park models are ordered from the factory and the dealer is more of a broker, but he may have a sales lot for other RVs or vehicles.
Here is one place that sells them and posts prices online.
Inventory for Sale : Park Trailers : Kropf : Log Cabin : Forest River : Used Park Models : Suncoast Trailer Sales
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL