My vote would be for your hourly rate (if you don't have one, divide your package price by the number of hours it would take you) times however long it would take you. I have developed a detailed pricing guide for 6 different types of vehicles based on size and have add on's and deductions for every service that I offer based on my hourly rate. If a customer wants to add a clay or subtract a shampoo, I always have a price ready.