Right now Direct2Drive / Direct2Drive UK have a promotion code for 20% off, which expires today. Use that promo code on BTK and its the cheapest price for the DLC around.
The promo code for D2D US is SURPRISE20.
The promo code for D2D UK is FESTIVE20.
http://www.direct2drive.com/11782/product/Buy-Battlefield-3:-Back-to-Karkand-Download
The promo code for D2D US is SURPRISE20.
The promo code for D2D UK is FESTIVE20.
http://www.direct2drive.com/11782/product/Buy-Battlefield-3:-Back-to-Karkand-Download