Cheapest I could find for this particular drive. Please don't post links to cheaper other brand drives and vote this deal cold as a result.
Buffalo 500GB JustStore Portable Hard Drive
Back up your files, free up some of the space on your computer and easily transport your files, with the Buffalo Juststore 500GB Portable USB Hard Drive.
Cheapest I could find.
This compact and lightweight storage device is capable of holding 500GB worth of videos, music, photos, files and documents, taking a large amount of data off your computer’s hard drive and helping it perform more efficiently. The Buffalo Juststore 500GB Portable USB HD works with Apple Macs and PCs via USB; there’s no need to install any additional software. It’s compatible with high-speed USB 2.0 ports too, which helps it to achieve up to 20% faster transfer rates