You might be able to provide comparable nutrition for less money just by switching from oats, for example, to corn. In many parts of North America, corn is a less expensive grain, and it is perfectly suitable for adult horses with good dental health. Also, because it's more energy-dense than oats, you might find that you can feed less and save yourself even more money. Once you breach the protective seed coat, vitamins within the grain break down quickly, and the grain becomes vulnerable to mold. Therefore, processed grains have to be purchased and stored in much smaller batches.
Processing also pushes the price up.
Even the source of a grain sometimes can influence its value, and even open the door to a possible bargain buy. Western oats usually are larger, plumper grains, and go for a premium price. Ontario oats, while smaller and skinnier, provide perfectly comparable nutrition, and often are much cheaper. When you comparison shop, however, make sure that the cheaper offering isn't cheaper because it's full of dust, fines, debris, weeds, or other undesirable products. It also pays to do some comparison shopping when you're buying hay.