My pony Misty is coming 10 years old, was thought by the prev. owners to be a haffy (she's not), bomb-proof, and will only walk/trot under saddle. (I personally also liked the idea of a pony my son couldn't accidentally get into a canter). She's sound, healthy, no arthritis, and was up to date on vaccs, wormings, and teeth. I paid $750 for her and as far as I'm concerned got the steal of the century. I'd say if your friend's pony is older, but the arthritis is only a problem in a canter, that $750 probably a fair price for her.