We currently have 8 horses on our farm and this is what I buy every week. Roughly 700 pounds of feed, 1 bale of hay and 1 bale of alfalfa. Most of the horses eat pasture hay though.
Horses kept in the barn require baled hay and shavings for their stalls. Currently we have 2 horses living in the barn. Each bag of feed costs about $10.00 - so it costs $18.00 per week in feed for each horse. There are cheaper feeds, but the one we use works best for our horses.