If you pay using your Egg card and go through Quidco it can be had for £30. Price on site is £39.99 for the Pre-Order. Egg are offering 20% cashback from CD-WOw and through Quidco you receive back 5%
Not ideal for everybody but if you have an Egg card and quidco account it's the cheapest I've seen for the much awaited game.
[The epic saga continues with Halo 3, the hugely anticipated third chapter in the highly successful and critically acclaimed Halo franchise. Master Chief returns to finish the fight, bringing the epic conflict between the Covenant, the Flood, and the entire human race to a dramatic, pulse-pounding climax.
Taking full advantage of the power of Xbox 360, Halo 3 expands on everything that has made the Halo franchise great, adding a wealth of graphical, technical and gameplay advancements, as well as offering a whole new level of online gameplay.]