Only £7.49 until then for the digital download. This version is different to the retail as its a special build which contains GFWL for achievements & it also has Nvidia's PhysX integrated into it (retail needed a patch).
http://www.microsoft.com/games/en-us/games/pages/sales.aspx
http://www.microsoft.com/games/en-us/games/pages/sales.aspx