CowBoom offers the used Microsoft Xbox 360 Slim 320GB Console for $104.99. Coupon code "x36073015" cuts that to $79.99. With free shipping, that's $64 under the lowest total price we could find for a refurbished 250GB model and the best price we've seen for a 320GB model.
Note: No warranty information is provided. These used units may be missing parts or accessories, including controllers or power cables, and have dents, scratches or other cosmetic blemishes.