The OEM M Performance one is pretty expensive. I'm thinking about buying/installing a Wavetrac diff. At the end it will cost you about $600+$700-800 deposit (core charge.) After you send them your stock core, you'll get that deposit back.
The cost of the install will be about $300-400.
With the M performance you can only put power to the ground on 1 wheel. With the Wavetrac, you can put power down to both wheels.
http://www.2addicts.com/forums/showthread.php?t=1033184