GfWLM has Rockstar games 75% off until June 27: http://www.gamesforwindows.com/en-US/rockstargames
Microsoft being Microsoft, be prepared for regional restrictions.
I have GTA IV on Steam. I should be able to buy the DLC on here to use with my Steam version, right? Cursory Googling seems to suggest 'yes', but if anyone has experience with how GfWL does DLC, 'd be great to know.
Awsome find, really wish I had more Microsoft Points...Not sure what I am gonna grab from this, but thanks for posting!