Trying to Work Out a Deal to Lease a Horse

So my dad's friend owns a horse that he doesn't really want, he would willingly give me the horse for free but we just don't have the money to board him somewhere. Although he's a friend of my dads he doesn't really want me riding the horse for free because then the horse is basically mine and he's paying for (technically) my horse.

So I approached him with the idea of a free lease and he agreed, if I pay him $300 a month. That may seem like a good deal but we just don't have that money, plus I just want to work with the horse twice a week (that's as often as I can get there plus my regular riding lessons) and $300 dollars for riding 8 times a month is a little ridiculous because I would be riding for 30 minutes.

Does anyone have any suggestions on a nice amount to lease he horse for that I can ask him about? Any kind of deal that would work out?