Presumably to celebrate the launch of Rage this week, Microsoft have reduced the price of select Bethesda DLC and avatar items. Also included in the sale are two of their XBLA games; Doom II and Quake Arcade Arena. Both are down to 400 MSP from 800 MSP. However this 50% reduction is only valid till October 10 so be quick.
Many thanks to Victor Sagat for pointing this one out.