I would say the best thing for them is the natural stuff made by God....but if you are looking for something man made that is "best", I don't think you can go wrong with Antlermax. Little more expensive than what you are feeding, but might be able to get a bulk discount buying as much as you do...that would be nearly 10 pallets of feed so you might be able to get it down to 10/bag price.
A local feed store or mill may have some good protein you could get in bulk orders for cheaper as well. 10 pallets is a decent order. most of the protein feeds are similar. not all made the same but similar.