i agree with JP on your worth and a form of agreement, i would also spell out in there the inerant risk of loosing colonies. Whose responsibilities if bees die in winter for whatever reasons? Do non-beekeepers know about that ? Certainly not and they'd surely blame the keeper for not taking care of their bees.
I think $100.00 per hour is what I would charge. For that price any problems and headaches are covered.
Definitely, definitely definitely have something in writing spelling out your responsibilities and don't fail to mention how bee keeping is a form of farming, dependent on many weather related variables.
You don't want to be responsible for a bad crop that is beyond your control.
What happens if the bees need to be fed?
I say they pay for ALL expenses including feed costs and your fuel costs to and fro.
Please keep us updated as this sounds very interesting albeit if you have the time for it.