I'm not in this line of bizness at all, but I think the easiest and most transparent way to do it would be to charge per hour of machine time plus a transportation/setup fee. you've got an hour meter. just document the **** out of everything with a digital camera before and after. nothing fancy, but better than your average cell phone camera. a quicker job doesn't get it all as good, but everyone should realize that going in. If they want a set price thell 'em I'll grind it up for X hours and you get what you get. its all about up front comunication.
a piece of paper with names, signatures, rates, dates, location, deposit and an agreement to pay the remainder after completion should be about all that you need.