Amazon is currently running their 15 Deals in 15 Days of Video Games deal. Every day there's some new video game related deal, today's just so happens to feature the very co-op friendly game Saint's Row 2. In fact, the best deal is the collector's edition, which is available for $49.99 (regularly $79.99). Also available are the regular editions for cheap, the PlayStation 3 version will set you back $42 while the 360 version will set you back $45.
Use the above links when ordering to support Co-Optimus, and don't forget to to read our review of the game.