According to a comment made by Major Nelson on his official blog, the Ninja Gaiden II DLC will be available for purchase off the XBLM at the cheaper price beginning Monday, March 2 and will remain discounted until Sunday, March 8.
As is with all Deal of the Week offers, only Xbox Live gold members can purchase the marked down content. Something that may have bothered Live silver members last week when Braid was only $10, but probably not so much this time. Seven smackers for the Mission Mode DLC is still mighty expensive. Especially for a game whose all-star ninja has been off the gaming radar for quite some time.