For today only, as part of Steam's Total War sale, you can nab both Shogun II and the stand-alone expansion Fall of the Samurai for 75% off their usual retail price on Steam. Meanwhile all the DLC released by The Creative Assembly for Shogun II and Fall of the Samurai is discounted by 25%, and so are not part of the Shogun II and Fall of the Samurai daily deal sadly.
Still, if you have been waiting for a good opportunity to join in the action, especially with the very recent release of Steam Workshop support and expanded mod tools for Shogun II, then now is as good a time as any. Do not forget you can still grab the Total War Master Collection on Steam through the sale with a massive 75% discount. That is some massive value for the usual Steam retail price of Total War: Shogun II.