Steam for Linux is out of beta, and to celebrate there's a discount on
Linux games on Steam until february 21st. The sale includes
Crusader Kings II for 10 euros. Note that if you buy that version, you get to play it on Linux, Mac and PC (so even if you don't have Linux, you can still buy it and play it on PC).
I cannot stress enough how awesome Crusader Kings II is. Okay, it's not Europa Universalis, but the game is excellent. The DLC is also on discount, but you don't need to buy it. Unfortunately the new Republic DLC isn't on sale, as that seems like a completely different game.
So.
Buy it.