I need to know the expenses of owning a horse?

Okay. well, I'm 13. I have a job, And will probably make like $2500 this summer. I have a friend who has her own barn in her backyard, no boarders. But they have an extra stall, and I was thinking i could board there like, really cheap. maybe $300-$400 per month. I want a horse pretty bad, and I've been around horses since i was 4, I know everything there is to know. I don't think I'd be getting a horse (if i get one) until I'm in 9th grade. I'm going into 8th currently. This way, I could take my friend's bus over to her barn after school at like 2:15, and my mom could pick her up on her way home from work at 4:00. I think its a pretty reasonable plan. I want to know the usual expenses of owning a horse, like, the basics. But how much do you usually pay a month to upkeep your horse? Everything. also, do you think my plan is alright/ do you think it could happen? Her house is only about 10 minutes from my house, also. So, tell me your opinion, because I'd really like to know! (: I don't know if this is even going to happen, but it could. (: