I think that is more than a fair price. Depending on what the cost is on materials, how far away is the business from your shop, because of fuel costs, and vehicle wear which shouldn't be bad you did great in figuring this proposal out. There is no rule of thumb that I am aware of. Any repairs that you find along the way would be on a service order , and of course I know you are aware of this or you wouldn't be in business long. Good Job, Rick & Sons.