As they say, buying the horse is the cheap part.
What you could look into is some kind of adoption, especially with OTTB. I've noticed that some really nice, young horses are sold for under $800 on certain adoption sites, like New Vocations
I wouldn't recommend this if you have little experience, but you sound like you know what you're doing!
I'm 16, and I have two horses. One is a AQHA gelding I got when I was 12, and the other is a yearling filly I was given this spring, for free. I pay board, which ends up $450 a month for both of them.
I'm in between jobs, but I've been able to work for the BO and get some board payed off.
Basically, just keep your eyes peeled! Sometimes it doesn't end up any way you'd expect with horses, but it doesn't mean it can't be great!