How much should you charge to wash cars?

That would depend on what the "average going rate or customary and acceptable rate" is, in your geographical location. To find that out, call around to car washes in your area and ask prices for the type of wash job you are specifically talking about (such as by hand or by machine, with or without wax, just body, or body and interior, undercarriage or not), then add the prices from all of them you call, & divide that dollar amount by the number of places you got prices from. The answer you get is what is what you should charge.