How about tor on a WRTG54 wireless router? That would be cheap, and I would guess it's possible given the linux OS firmware. I can't make this happen since I'm not a programmer, but the idea has occurred to me before. I like the HyperWRT project, Thibor version. (http://www.thibor.co.uk/). Maybe start a thread in the forums to see what the response is.
-P
Thanks,
Brad