I think by the hour is the only fair way to do it. That eliminates the variables of block size, desired split size, knots, etc.
I have not done this, but I'd probably fire up the tractor and splitter for about $40/hr, plus mileage. Mileage charges would be exorbinant after about 5 miles, though as it's either take a scenic tour on the tractor, or find someone to haul it. Helper required to be provided on all blocks over 20", or I will bring one for $15/hr.