In the condition he is in there I would pay maybe $500. (US)
He really only has basic training, and probably could only be used as a trail horse and pet. He is unregistered which brings his value down significantly.
If you cleaned him up, and put a bit of weight and muscle on him, I think you could advertise him around the $1000 to $1200 mark, depending on how the market is in your area. Also be careful how you write when you place your add, and make sure you are labeling him correctly, ie. Saying he is a leopard appaloosa when he clearly isnt.