The shipping companies will typically base your fee on how far the horse would have to travel if he were the only one on the trailer. He might actually travel much, much farther than that however. Say they have 2 horses in IL that are headed W one in the southern part and one in the N. The horse that was picked up first might ride around and extra 100-200 miles to pick up the other one but you would not pay for that travel if it's not on the way. They will have loads going back and forth across the country so it would only be one way.
I see rates around $1/mile but they could all have fuel surcharges tacked on.