Gamestop put out their own DDR pad--they sell it for $20 at their stores. They feel pretty cheap and they're not responsive enough for good players, but they work for somoene who just wants to mess around with DDR.
I wanted a cheap pad awhile ago too, but once I started to play DDR more and more I wanted a better one. I ended up paying about $70 for a Red Octane pad that's really nice.