Well, I assuming you keep your horses at home like I do. Last year I switched to using round bales and cut my hay bill by over half. I also make my own slow feeder round bale net, which makes the hay go even farther.
My horse is an easy keeper, so I put him on a ration balancer.
(There are some good threads here about ration balancers.)
He basically gets less than one pound daily, so one bag of feed goes a loooong way.
I do give him an expensive hoof supplement and flaxseed, but all in all my food totals on him are less than $70 a month.
That saves me quite a bit.