Found this on another site, you can pre-order Borderlands 2 Season Pass for $22.50, sells for $30 normally or $10 each. This gives you all 4 pieces of dlc planned for Borderlands 2. Comes out to $5.62 a piece.
http://www.greenmangaming.com/s/us/en/pc/games/rpgs/borderlands-2-season-pass-na/
Use code GMG25-1BW0K-K1A3G
http://www.greenmangaming.com/s/us/en/pc/games/rpgs/borderlands-2-season-pass-na/
Use code GMG25-1BW0K-K1A3G
Can't get enough Borderlands 2? Grab the Borderlands 2 Season Pass!
Includes $39.96 of add-on content and will be available for SRP $29.99
Includes four brand new individual add-on content campaigns for Borderlands 2
Each campaign will feature several hours of gameplay and introduce new adventures, allies, enemies and environments.
The Borderlands 2 Season Pass will be available for purchase when Borderlands 2 is available on on September 18, 2012.
All four add-on content campaigns are scheduled to be available by June 2013.