The app markets are filled with so-called "free-to-play" games, and it's no wonder: the most popular free games can rake in millions of dollars per day. These apps generally make money through a combination of advertising and in-app purchases. However, a recent survey found that about 0.15% of mobile gamers are making 50% of all the in-app purchases in free-to-play games. Read on as we take a hard look at this trend in mobile gaming.