20,000 is much too much for a not purebred with no papers and confirmation faults. Particularly since he cannot be bred and has a history of a health problem. Due to his schooling and height and age I would say he could be worth 10,000. But that depends on the area you are in. Since he is so tall that is a good thing as tall people are willing to pay for a horse that fits them. Right now is not a good time for horse values as there are many people selling who cannot afford their horses and that brings down the value of your horse as well.