For one day only, July 5th, the Xbox 360 version of Mega Man 10 will be marked down from 800 MS points to 400 MS points (5 dollars) as part of the latest "Deal of the Week." That's half off its original price! Of course, there is a bit of a catch: you must have an Xbox Live Gold membership in order to purchase.
Again, it's a one day only deal so jump at the opportunity while you can. More information on Deal of the Week available here.
Thanks for the tip, Hardcore Hecxz and Mr. Flint!