OfficeMax offers the Hitachi Touro Desk Pro 3TB USB 2.0 External Hard Drive, model no. 0S03290, for $119.99. (Proceed to product page to see this price.) With free shipping, that's $0.04/GB and the lowest total price we could find by $20. Sales tax is added where applicable.

Update: Specs corrected. (This is a USB 2.0 drive, as Hitachi notes on its site. OfficeMax erroneously refers to it as a USB 3.0 drive in its specs.