There is a site called HitBliss where you can watch ads to earn money which can then be spent on TV shows, movies or a monthly Pandora subscription. An important aspect of HitBliss' model is that people who do not want to have ads can buy stuff outright and the ads are completely separate from the content.
HitBliss has said that they would like to work with other companies. So my suggestion is that you work with HitBliss to incorporate a way for people to pay for cash shop items or their subscription with the money earned via HitBliss. I believe this would add a lot more liquidity to the game since people can watch ads for the content if they choose or they can pay directly.
If I wanted to watch an ad, I'd look at an in-game billboard. If I wanted to listen to an ad, I'd tune in to The Cape Radio.
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I believe you are missing the point. It wouldn't put ads in the game, but would be a way for people who either don't have money to spend directly on the cash shop or subscription to pay for it another way outside of the game.