That's a loaded question. It really depends on what you're trying to accomplish here. Are you thinking of a shop or mobile? So with a shop you will have monthly rent to calculate and can use water and soap to wash. If mobile, you need to figure out how you're going to wash the car (pressure washer or customer's water). You will need a van or truck of course. Assuming you have those things covered, maybe another $2k for materials. But, it really depends on what you're going to offer and where you plan on doing it.