In a letter released today THQ announced that Metro 2033 will be half price from 16th October:

"For players who want to get ready for the 2013 launch of THQ's highly-anticipated first-person shooter Metro: Last Light, they can soon purchase the critically acclaimed, cult-classic Metro 2033 for half-off on Xbox LIVE. Metro 2033 will be available for 5.49 beginning tomorrow for a limited time only. The deal ends Monday, Oct. 22 so download it now!"