I'd recommend looking into ration balancers- they look more expensive at the price per bag, but are fed in much smaller amounts than regular grains so the price per feeding is actually lower.
For example, at my local feed store you can buy Safe Choice for ~$15 for a 50 lb bag. For maintenance of a 1000 lb horse, it recommends 2.5-5 lb. That comes out to $0.75 to $1.50 per day.
The same feed store sells Empower Balance (also from Nutrena) for ~$22 for 50 lb. The feeding recommendation for the same horse is 1-2 lb. That's only $0.44 to $0.88 per day. You're not skimping on quality or nutrition, it's lower starch than regular grain, and you could even feed it all in one feeding if you needed to. Purina also makes a ration balancer that is even cheaper (Enrich 32).
Ration balancers are, however, not going to provide as many calories as grain. If any of your horses are hard keepers, you may need to supplement with beet pulp, rice bran, or alfalfa pellets/hay/cubes. These are all typically fairly inexpensive, and you only need to feed it to the horse(s) that need it.
I second Loosie's suggestion of FeedXL.com- I found it very helpful for my horse. You can enter in the costs of the feeds you're considering and it will show you how much it costs per day along with how well it's fulfilling your horse's needs.
And here's a good resource for learning more about ration balancers: Ration Balancer