More than 3 million Xbox Live subscribers pay a $50 annual fee that allows them to play games online against one another. About 3 million more use the service without an annual subscription, taking the total to more than half Xbox users. Through Xbox Live, players can buy maps for fighting terrains and other add-ons that are indispensable to serious gamers, usually at a cost of $10 each. Such add-ons used to be free most of the time and the additions can raise a game's cost to $80 or even $100 over its life span. Wedbush Morgan analyst Michael Pachter estimates Microsoft charges a royalty rate of 30 percent for most downloads. That means that, for a $10 download, Microsoft gets $3, while the publisher pockets $7.

But pricing the downloads is a tricky science that many game publishers, including Microsoft, are still figuring out as they try to balance profit with the risk of alienating gamers who might feel they are being taken advantage of. Microsoft was criticized by many gamers after the May 3 release of a map download to its popular alien-blasting game, "Gears of War," which has sold more than 3.7 million copies worldwide since its November release. The first map pack was released for free months earlier and generated more than 1.5 million downloads. The new download included several new battlegrounds online users could fight on during multiplayer matches, but this time they cost $10.