The Single Player DLC will be "Lost Archive" and it'll cost 1800 MS Points/$14.99 for the Xbox 360 and PS3 respectively. It's unconfirmed to come out February 28.
Source: Kotaku: http://kotaku.com/5886386/assassins-creeds-next-dlc-is-lost-archive-says-listing
EDIT: Kotaku corrected it to 1600 MS Points, though it should 1200....
EDIT 2: IGN confirmed from a blog post that it'll be out for the Xbox 360 at 800 MS Points. PS3 will most likely charge it at $10.
IGN: http://xbox360.ign.com/articles/121/1219110p1.html?utm_campaign=twposts&utm_source=twitter
EDIT3:
http://www.youtube.com/watch?feature=player_embedded&v=O6Fsu_tsweo
An email to GameStop customers on Friday named and priced the next downloadable content extension for Assassin's Creed: Revelations. It is called "Lost Archive," and it will be $14.99 or 1800 Microsoft Points, for both PS3 and Xbox 360. The email said the pack would drop on Feb. 28, but that is unconfirmed.
Source: Kotaku: http://kotaku.com/5886386/assassins-creeds-next-dlc-is-lost-archive-says-listing
EDIT: Kotaku corrected it to 1600 MS Points, though it should 1200....
EDIT 2: IGN confirmed from a blog post that it'll be out for the Xbox 360 at 800 MS Points. PS3 will most likely charge it at $10.
IGN: http://xbox360.ign.com/articles/121/1219110p1.html?utm_campaign=twposts&utm_source=twitter
EDIT3:
http://www.youtube.com/watch?feature=player_embedded&v=O6Fsu_tsweo