If you say that, you probably won't get much business. You should try explaining what exactly you do and maybe give a small demo so they can see you're not trying to rip them off. Explain your process and how long it will take you depending on the condition of the car(s), and what you're going to charge.
I'm not in the business, but I'd think keeping your car in pristine shape as a demo would sell more than anything else. If it's a dealership or something, you should make the point that a well-detailed car would have more initial "grab" from the road and also could be sold at a higher profit. Think about what they'd be gaining from it...more than just clean cars.
