I would say that $13/ton is a bit high as well, our guy is $10/ton but we have about 4 different custom grinders around us.  In good alfafla he grinds about 80 ton/hour. He runs 2 grinders, Haybusters, mounted on trucks and I think they are 600hp each. Trades one off each year, IIRC he said that a new one blows through $1/2mil. Ill keep hiring it done, hes always on time and we dont have the headache of upkeep.

Edit: I think in cornstalks hes a little higher because the stalks grind slower and are harder on the grinder than hay is. Not sure what he charges for them though.