When I was boarding my saddle horses, self-board, I figured 3 ton per horse per year (dry lot situation, so fed hay year round). It came out fairly well to meet my needs, they got 1.5-2% of their bodyweight in hay per day (a little more in winter, a little less in summer). The minis, I figure about 1 ton per mini per year.
We raise cattle, and do put up our own hay, but most is not suitable for horses (too much alfalfa, or too coarse), so I still have to buy some hay for the minis (and my senior big gelding, the other saddle horses do fine on our homegrown round baled hay). I buy small squares for the minis and supplement it in winter with our homegrown hay in round bales. Last year, my FIL put up a bunch of small bales for me, but the minis don't care for the type of grass in them, so this year I'll be buying their hay, so I don't waste as much. [Thank goodness for the cows, they'll eat anything the horses don't, so nothing goes to waste; but I hate to pay for hay that ends up going to the cows, so really need to watch the quality of what I buy.]