If you have serious work to do renting a unit is good advice. Good quality units can go for $2500 or more. Industrial units, capable of stripping paint, removing rust, and etching concrete go for many times that.

Most people just don't do enough with serious work with a pressure washer to justify the cost of a good one. The Sears/hardware store low-pressure units for under $100 are okay for occasional light work, most of which could be accomplished pretty well with a garden hose, are okay but expendable.