GetGamesGo (Eurogamer's store) have Shogun II up on pre-order for only £23.99. Until the 15th you also get a free copy of Rome: Total War!
According to Find-Game this is the cheapest price currently available, and that's before you take in to account the free copy of Rome (which TBH I have no use for, already own it).http://www.find-game.co.uk/28179-Total-War-Shogun-2-Limited-Edition.htm#B004GHMUAQ
Worth noting though, this is a downloads store. Buy from here and you'll have to D/L all 15GB+ of the game!
Still, best price about right now and way cheaper than the Steam version (while being the same, all copies are Steamworks, you could even download from Steam instead of GGG!).
EDIT: Enter code 'romansamurai' at the checkout for a further 20% off, making it £17.99 for Shogun II AND Rome! Yowza!