Worth is a hard thing to estimate. It pretty much depends on the location of the tractor, the condition of it, and the maintenance history. Considering that there aren't many of these around, you have to go by the engine size, and the deck size too. Check your local Craigslist postings and see what other brands are going for in your area, and you can basically go from there. The thing that depresses the price is, as you said, rarity and cost of parts...
Rattosh51
YT6800