I don't think this is exactly what you are offering, but it might give you an idea on pricing. When I was in high school my friend and I gave pony rides to young children. We charged $10 or $15 (i don't remember which) for a 20-30 min ride. We would have the pony tacked up, and the child would get to groom on the pony for a few mins before and after the ride. 20 min rides were the best, the 30 min rides were a little long.