Pre- Order Dead Island Riptide DLC: Survivor Pack (PS3) For $43.80 (shipped) @ The Hut
This is the best price I have been able to find for the PS3 edition. Zavvi is offering it at the same base price but without the additional 10% off so the overall Zavvi price is currently more.
Both The Hut and Zavvi are selling the PC edition for $30.70 (shipped) and the XBOX 360 edition for $48.45 (shipped). No promo codes apply to the PC and XBOX 360 edition on either Zavvi or The Hut.
The release date for this item is 26 April 2013.