Go with an hourly rate, figure out how much it costs you to travel to teach & use that to base your 'travel expenses'. Most teachers I know charge a rate that combines the hourly rate with travel time so each person may get a different rate depending on how far you have to travel. Since this is what I observed, it's what I do myself.
As for how much to charge, well only you know how much you think you're worth. Charge accordingly.