Just a quick internet search the prices run from $15000 to $25000. I can't really explain the wide difference in price. Hopefully someone more knowledgeable on Kubota tractors will come along soon.
Would be a good place to start estimating what the tractor's worth, in my opinion.
Location can make a difference, as well as condition, accessories, transmissions, etc. Some areas like green, some kinda like orange better, etc.