Well, instead of the Ultium you could use a combination of beet pulp, alfalfa pellets, and flax meal. You'd need 1 lb of beet pulp, 1 lb of alfalfa pellets, and 1 cup of flax. That should provide all of the extra calories, protein, fat, and nutrition that the ultium provides, and it should work out to be cheaper. Alfalfa pellets cost me about $13 for a 40 lb bag and beet pulp is about $12 for a 50 lb bag at my local tractor supply. One bag of each will last more than a month, so you're looking at $18 a month for those two ingredients.
Horsetech.com has a good product called NutraFlax. Their prices include shipping. You can even have them custom blend any extra goodies you might need in it, like joint supplements, pro/pre-biotics, yeast cultures, etc. If you buy just the NutraFlax, in a 20 lb bucket, you're looking at a $16.50 a month cost. You could get whole flax ordered from your local feed store for even less.
All that said, looking at the feeding recommendations of Equine Senior, you're not feeding enough of it. For a 1,000 lb horse in light work, you're supposed to feed 14 lbs a day. Why not increase your Senior to 5 lbs twice daily?