Crusander Kings 2 is $10 at Gamefly today. Tempted, but there's the DLC and then it starts to add up…
CK2 on its own is well worth it, you don't need any DLC at all. The Sword of Islam expansion basically lets you play as the musulman countries with a different political system, but if you don't care about that, you won't be missing anything. Besides, there are very good player-made mods for CK2 (the Game of Thrones one is really really really really really good for example)