Costs vary by area quite a lot, and I have been researching the same question for a little over a year. IF you don't have to board, the cost will be very close to 200 dollars a month after you pay for the horse and all of your tack. Boarding usually includes some feed, and in this part of Texas runs 175 to 250 dollars for unfancy, but good facilities. At least the cost of horses is way down right now, so that helps a little up front. Highly trained horses in specific sports seem to be holding their values better than trail or riding horses.
Too bad it is so expensive, and seems to be going up pretty steadily. More horses are being abondoned now, and most recovery shelters are overwhelmed.