this time around the content is going to cost you. The special part is continuing the aquatic action won't cost 400 MS points as originally expected because it's currently available for 200 MS points ($2.50 in surface currency). If this is an accident and you're a fan of Undertow, now's the time to purchase it!
If the half-price sale hasn't sold you, here are some details on the Path of the Elect expansion. It includes five campaign levels, a new alien race called the "Elect" and four multiplayer maps. Now, go grab this deal in case it doesn't last.
Update: The expansion price has been changed to 400 points.