How much will this horse cost monthly?

So,I'm getting this horse, and boarding from my landlord. The fee is $75/ monthly including hay. The horse is about 4 and is a quarter horse. So in a horse owners opinion how much would I be paying monthly for feed. Its going to be pasture boarded. also any thoughts on negotiating a better deal. I mean this already is a pretty good deal, but I am 14. So, suggestions on making more money? Thanks, and please no lectures.