The first DLC called The Hidden Secrets has now been released by Ubisoft for Xbox 360 and PC Season Pass users.  If you are on PS3 then the DLC will be out tomorrow and if you do not have a season pass then expect to pick up The Hidden Secrets on December 11, and for PS3 users, make that December 13.

The Season Pass for Assassins Creed 3 costs $30 but if you do not have one there is no telling what the DLC will cost by itself, although the $10-$15 price range would make sense.