I can't see this having been posted yet and the cheapest price I can see to date is £19.85 at simplygames.
If it's available cheaper elsewhere, I'm sure I'll be looking foolish soon enough!
* Meet Noble Team. For the first time, players will fight alongside a squad of iconic Spartan soldiers, each with deadly talents as unique as their individual personalities and customised gear
* An intense campaign. Live the events that set the stage for the Halo trilogy as the UNSC and Covenant clash at the height of their military power
* Stunning technical advancements. Halo: Reach takes a massive leap forward through all-new engine technology designed to take full advantage of next-generation graphics, audio, special effects, AI and animation
* The definitive multiplayer experience. Halo: Reach builds on the success of its predecessors, setting a new standard for competitive gameplay, customisation, variety and community integration
* Unparalleled feature suite. Halo: Reach expands on the industry-leading suite of features found in Halo 3, including four-player cooperative
Also worth making sure you have 800 Microsoft points so you can buy the Noble map pack, if you intend to play online.
Also, I've discovered it's also this price at play.com. Some people prefer Amazon's service, but for those that want the quidco- you're covered too :)
Play.com link: http://www.play.com/Games/Xbox360/4-/17659541/Halo-Reach/Product.html?P36=SVHXBS