I went though Costco which uses Authorize.net. The base charges are 20 per month for the gateway and merchant service- 40 total. The percentage is around 2.7 of each purchase. 95+% of my sales are online, so that is different than daily local detailer situations. For local, I paid $10 for the ol' style of credit card machines that "slide". I give the customer a receipt, go home and process the card manually. Since I only do about 10 per year, it's not a big deal.
If your customers are not asking if you take credit cards, spend the money on marketing/advertising each month. As you grow, add the cc service. My brother does weight balancing, prop balancing, and detailing for airplanes. Charges start at around $250 for just one of the services. He does not take credit cards and customers pay every time with cash, or checks.
Rob