In-Game Purchase, Are You the Generous 0.15%?

(2/28/2013) Most of mobile games on Apple's iOS and Android are free. To compensate the game development cost, the gaming companies offer in-game purchases to allow players to pass faster, get tips to receive 'treasures', or get 'one more chance' to defeat the enemy. But, how many of them are paying? Not many, only 0.15% of gamer ever pay for in-game purchase but this part counts around 50% of revenue for those game developers. If you are a game developer, how can you design your game to be business sustainable?