Craigslist has some cheap servers, but not energy efficient. The old amd 940 opterons use 55 watts and go for less than $10 at some venders. The boards were being closed out at computer geeks for around $24 shipped for awhile, but they sold out. Make someone an offer and you might get a bargain, but it won't be low energy. Some of the fancy dual core atoms with board go for almost $200, so figure the initial investment versus energy cost savings if you can.