Gamerzines, ME2 director Casey Hudson described the DLC as "five levels set across one story on one planet," with players battling through a laboratory (and occasionally piloting the Hammerhead tank) to defeat an artificial intelligence. There is currently no price nor official release date.
Hudson also announced that no new squad mates would be added in the DLC, nor would there be in any future DLC. We'll be sure to update as soon as EA makes a formal announcement with more details.
Update: The official Mass Effect 2 website has posted a more detailed description of the scenario (reproduced after the break), along with the price: 560 MS/BioWare points ($7).