The amount you're asking is under the half-lease rate at the stables where I ride. And the half-lease is 3 days of riding plus a lesson (or a lesson discount, I'm not sure - I don't lease there). If someone wanted to lease a horse to me for 5 days riding per week plus any shows I wanted to cart him off to, and I wasn't going to have to pay farrier and vet, and all they wanted was $300 a month, I would find that a very attractive deal. (Assuming that the horse was a good fit for me, that is.)
I think this must depend a lot on where you are...I'm in New England, which seems to be more expensive than many other places.