Well, the main food of a horse is pasture or hay, and hay prices vary greatly depending on your area and type/quality. For a sqare bale, I've seen it anywhere from $3 to $15+.
As for feed, most of them are in the $15/bag (50 lbs) area these days, and how much you need depends on how much you work your horse and how much pasture/hay they get. A healthy, lightly ridden horse with acess to pasture/hay really doesn't need any feed at all.