It's amazing how difficult it is to sell an iOS game for anything more than $1.99. But, if you give them the game for free and let them exchange real money for strategic advantages in-game, they'll fork over a small fortune. Supercell is essentially selling $2.5 million/day in "gems" for only 2 games, gems that people then trade for resources or the ability to do things faster (like building units or structures).
A user named 'Panda' on the Supercell forums claims to have spent over $6,000 on gems in Clash of Clans[1]. Based on the responses regarding how quickly he was able to reach the top of the rankings, it seems to be true. Many other players in the thread report having spent $20-$40 on gems.
[1] http://forum.supercell.net/showthread.php/827-How-much-money...