I raise all my own hay and oats so what the cost is varies somewhat and with 40 to 50 head if I bought it then the price would be pretty high. But the best way I've found to put and keep weight on a normal healthy horse is to give it all the good quality hay and or grass it wants 24/7. Horses are forage animals and mother nature didn't design them to eat grain thats something people decided they need. Really good forage will beat any grain or feed there is. A couple lbs of grain is fine for your average horse but the forage is the key to proper health and weight.