Just picked up a Greenworks 1700 psi(~$190), seems a tiny bit lacking in power, but I probably wouldn't go above 2200. Using it with a nice $20 foam cannon and the results are pretty nice for how cheap the set up was, and I didn't even use a dedicated foam soap yet. Also, one of the $100 foam cannons would likely work better. Here's a pic from the cheap set-up of an E46 M3 I detailed a couple days ago:
