Mark of the Ninja from Klei Entertainment – one of Joystiq's favorite games of 2012 – bringing it down to 600 MS Points ($7.50).
Other cheap contenders include Trials Evolution for 800 MS Points ($10) and some discounted Pinball FX 2 tables. There's also Dust: An Elysian Tale and Deadlight for 600 MS Points ($7.50) to consider.
On top of this promotion highlighting Microsoft Studios games, there's also the ongoing Xbox Live Arcade Deal of the Week sale. Retro City Rampage has been knocked down to 600 MS Points ($7.50) and Quarrel is 200 MS Points ($2.50). The full list of both sales can be seen through the source links below.