Used to charge hourly (with a one hour minimum), but when upgraded machine, starting working on a 'piece rate'.
Per stump, have a price per size category. Has been a much more lucrative way to operate the machine, particularly for those jobs with a large number of small to medium stumps. Certain stumps (difficult access, very large, particular species), a hourly rate may work out better, but I try to price those ones differently.
For a new Vermeer SC382, I aim to hit above $300.00 per machine hour. $100.00 per hour would be very skinny for the cost of that machine.