The new DLC is due to arrive on the US PlayStation store on the 27th of November and is priced at 1200 Microsoft points or $15 (expect it to be around the £10 mark in the UK).
The DLC allies commander Shepard with space flapper Aria T'loak so he can assist her in taking back the space station Omega, which is in the clutches of the evil Cerberus. The DLC is basically a directors cut, as it is content that was taken out of the released game, which explains Aria's tiny role in the main story.
Bioware claim this is the biggest expansion yet, but with it being set halfway through the story, it all depends on if you are willing to plow through again or not. You already know how the story ends after all.