I've just had a guy (a painter I know has done good work before) come up to my door and offer to power wash my sidewalk, driveway, walkway, and porch (all concrete) for $350. They're all quite mildewy and dirty. Anyone had this type of work done before? Does this seem like a good price? I've got about 150 feet of sidewalk, 60 feet of driveway, 50 feet of walkway, and a 30 square foot porch.