I've looked at doing this for years. I've came real close to buying a mixer grinder. My only problem is if I buy all the ingredients its a break even or even lose not counting my time. I go through a lot of feed also. I drive about an hour away to get cheap feed directly from a feed mill. Now if I had more time, land and money I'd probably grow most of the ingredients and store in bulk bins and mix as needed.
They make a nutri-balancer that has a lot of the necessary vitamins and minerals which is pricey. Its one of those things where you'd have to decide if it was worth it, if your making organic feed maybe it would. I'd love to but can't right now.