The price can change too much depending on your area, if he can compete, if he can compete and win...
Winning ponies, that can bring a young rider to big events, are pricey, even if they rear. Not-winning ponies, or ponies that will never have the chance to compete and prove themselves, cost way less.
What you have to do is ask the owner. He might not want to give up a good school pony, or he might be more than willing to have you pay for his care. Those things change the price a lot. I know it sounds unfair, but the "market value" and the asked price can be different just because you already want that pony, instead of just a pony.
Ask your trainer. Then your mom will decide what to do.