I'm paying about $190 a month right now, because I'm actually getting free board for my mare because I feed with another girl every evening for the barn (40 or so horses). The $250 comes from paying half-board for my filly ($100) and feed for both her, my mare, and part of the feed for another filly that I'm helping rehabilitate. Hay is included in my board thankfully, and they're pastured so no shavings or anything. Then theres the trimming every 5 weeks, worming, suppliments (just for the my mare's filly) and that sort of thing. Once sell my filly though in the next week or two, my costs should dip down to $90.
If I didn't feed every evening though and paid full board for both of my horses, I'd be paying a whopping $475 a month, and I'm only a part time paid 17 year old! Eeek.