You can go as cheap as the Hercules DJ MP3 ($130 or so) or something better in the $250-300 range like the Vestax spin.
The Spin cost me around $300 at the Apple store and it has by far the best scratching/jog wheel function for any of the controllers in the price range.
It's basically a VCI-100 with a few less knobs and a soundcard. They say it only works for Mac's, but it works for PC no doubt.