[Bundle Stars] has the "Crusader Kings II Bundle" for US$12.99, it's all Steam activated and the game does have Steam Trading Cards.
(Steam Link: http://store.steampowered.com/app/203770/?snr=1_5_9__205)
Below I've gone to the effort of listing the game (+ the 10 DLC) with the associated Steam prices. The game has a huge amount of DLC, which is the only issue I can fault it for, but that DLC often comes up on special.
Crusader Kings II: $39.99
Crusader Kings II: The Old Gods DLC $14.99
Crusader Kings II: The Republic DLC $9.99
Crusader Kings II: Sons of Abraham $9.99
Crusader Kings II: Sunset Invasion DLC $4.99
Crusader Kings II: Legacy of Rome DLC $4.99
Crusader Kings II: Customization Pack DLC $4.99
Crusader Kings II: Ruler Designer DLC $4.99
Crusader Kings II: Celtic Unit Pack $1.99
Crusader Kings II: Saxon Unit Pack $1.99
Crusader Kings II: Turkish Unit Pack $1.99
Total: $100.89
This is a truly awesome game, quite overlooked by most mainstream gamers. It's scored 95% on Steam and 86/100 on Metacritic, it's also one of my favourites. I'll leave you with 2 of the more amusing, but accurate reviews.
"At the age of 16, my character developed a mad crush on a local girl. He crept into her room and banged her, then declared he loved her. 9 months later she gave birth to a child, he decided he didn't like it, smothered this child in its crib, pushed his lover off a balcony and then married a horny Greek princess and did it all over again until he commited suicide at the age of 72 because his son turned gay after coming back from a visit to Constantinople. Brilliant!"
*by: Kana Sasaki
"I bribed the pope, murdered a whole family, including babies. Was granted the title 'the Holy'."
*by: Parkman
I've spent 500+ hours on this game, and have probably spent $200 on DLC. So that's a great deal, it's missing the two most recent major DLC's, but the game is so big and complex that it's probably for the better that you have less options to begin with.