mwave offers the refurbished Western Digital My Passport Essential 500GB Portable USB 2.0 2.5" External Hard Drive for Mac in Black, model no. WDBAAB5000ACH-NESN, for $59.95 with free shipping ($0.12/GB). That's the lowest total price we could find for this refurb by $15. It's formatted for Macs and is compatible with Time Machine; however, it can be reformatted for use with Windows and Linux.

Note: A 180-day warranty applies, although it's unclear who supports it.