The guys around here tend to charge X amount per man hour and Y amount per machine hour. Both charges are time out to time in since an hourly worker gets paid while traveling from job to job (except lunch break) once his day starts and the tractor or whatever is hauled by a dump truck and trailer that don't run for free.
When I hire out excavating, the guy I work with cuts me a small break since I helped him out with some work when he was just getting started. He hits me for $60 per hour, man or machine, but everyone and everything is working pretty hard during those hours. His normal rate is at least $65 per hour, more for the bigger machines.
You can get some idea what to charge by trying to estimate how many hours you expect to get out of your $30,000 backhoe investment, plus the maintenance/repairs on it during that time. Then figure what a good operator costs his company, including insurance, unemployment, office costs, etc. that accrue while he is working. The guy may get paid $25 per hour, but the cost to keep him working is considerably more than that.
I'd say $50 per man and machine hour is more than fair. Auto dealers charge more than that for a technician to work on your car or truck.