Make a mobile game that climbs near the top of the charts, and the dollars start flowing. In a big way. For proof, look no further than the booming business that’s fueling casual game developer SGN. Critical to a breakout mobile game is having tens of millions of users download the app and play all they want for free, with a small percentage of fanatics paying for advanced features or to skip levels. The 10 percent of paying Do paid mobile games make money users are producing pretax profit margins for the company in the neighborhood of 20 percent, at a time when scores of heavily funded tech start-ups are burning millions every month for growth. Read More Facebook’s top game aims to sweeten up China.