Recommendations for RMBP External Hard Drive

I am looking for a cheap external drive for my RMBP as a backup.

Some basic requirements include:

  • 250GB or larger size (I have the base 256GB RMBP)
  • Fastest I/O, USB3 or Thunderbolt preferred
  • Time Machine/OS X compatible
  • As cheap as possible
Anyone have any recommendations or experience with such an external drive?