1 Hiring a professional pressure-washer costs about $0.08-$0.35 per square foot (depending on local rates), but can go as high as $0.40-$0.80 a square foot depending on the amount and type of stains, dirt, mold or other problems. Buying a basic residential pressure washer with an electric motor starts around $70-$200, depending on size and capacity. 2  A gas-powered residential model usually has a higher pressure rating than an electric machine, and is about $300-$1,000.