My local feed store carries ground flax from Animed for $29 for 20lbs. I feed 1/2 cup to most of my horses, which is only 2 oz by weight. They do very well on it. My fatties only get 1/3 cup and my "hard workers" (that tend to lose easily) get 1 cup for weight gain/maintenence.
So, feeding 1/2 cup, or 2 oz, only costs about $0.19 per-day per-horse.