Three DLC packs are being planned for Dishonored with the first one dropping this December, to the tune of 400 MS points or about 4.99$ for PS3 and PC. The first pack will be ten challenge maps, in those are trails that will test your stealth and mobility. One of the maps will be an arena setting you up against waves of enemies.
The second DLC which is coming in the spring of 2013, will be all about “The Whalers” and their supernatural leader “Daud”. This will be a story driven DLC. Bethesda will release the price when it gets closer to the release. They also are not saying anything about the third DLC pack.
News Source (Eurogamer)