Halo is one of the most popular games for the Xbox console. Microsoft has just announced that the Halo franchise has hit 46 million sales worldwide. The 46 million sales generated around $3 billion in revenue worldwide.
Even though the $3 billion may seem low compared to games like Call of Duty, keep in mind that Halo has been exclusive to the Xbox the entire time. People have spent 5 billion hours playing Halo and the community has created 120 million pieces of content for the game including maps, screenshots, gameplay videos, etc.
These numbers are sure to surge after the release of Halo 4 (see the trailer).