Here's the math, presented in part of a longer open letter from gaming business analyst P.J. McNealy to Activision boss Bobby Kotick:
Gamer A's five days' worth of playing = 120 hours spread across a year. That's about two hours a week. Play twice that much per week and the game costs just two quarters per hour—not counting the console it's on, Xbox Live Gold if you're on a 360, and some other factors, of course!
You could apply this to any game, of course. And if you don't buy DLC and only pay $60 for a game, you can cut all these per-hour costs in half.
Why Bobby Kotick has been correct on Call of Duty and the video game industry [Digital World Research]