I feed my 31 yr old blind half toothless appy a pelleted feed called Total Equine. It can be fed from 2# twice a day up to 6# twice a day for horses who cannot eat anything else. It's not cheap tho and not available everywhere. If you can get it, I highly recommend it tho.
Other feeds I have and do use in addition to smaller amounts (2-4# twice daily) of TE.... Alfalfa pellets are great, sometimes you have to soak them if the horse lacks chewing power, ie molars. And also I highly recommend Beet Pulp. Shreds are best, imo as they provide better fiber that way. My old horse also chews around on prairie grass hay that I have cut up into 4" long pieces.... Yes it's a pita... I only give him around a flake twice daily as he gets enough poundage via the Beet pulp and Total Equine to sustain him. He won't eat the Alf Pellets anymor, because he's a pita. ;)
So the least expensive way would be adding Beet Pulp.. It can be feed free choice once they are eating it..