Nearly all of the top grossing iOS games and apps are free

"For every overpriced Final Fantasy re-release on iTunes there are dozens of free-to-play social games, refugees from a slowly fading Facebook love affair with microtransaction-driven casual titles finding new sources of revenue in the fertile mobile market. In this year's list of the top grossing iOS games and apps (available within iTunes), the free-to-play game is king.
Not one paid game in the top ten. In fact, the first paid game in the list, the Pictionary-inspired sensation Draw Something, comes in at number 16 on a list dominated by games you may have never heard of. Take Kingdoms of Camelot: Battle for the North, a kingdom-building strategy game that took the number one position this year. The top in-app purchase is a bundle of 240 gems for $19.99. While others quibble over the $15.99 price tag of a remastered classic role-playing game, thousands are dropping more on virtual items within a game with a much less prestigious pedigree.
The top ten list includes a wide variety of free-to-play tropes. The gambling itch is scratched with slot machines and poker. Addictive gameplay earns big in Bejeweled Blitz. Compulsive collectors flock to my beloved Dragonvale and the herald of the new card game age, Rage of Bahamut.
There are some 81 games on this year's top grossing iPhone app list (that doesn't include iPad apps, but the results are similar). Of those 81, 17 are paid titles. Four of those are Angry Birds derivatives (Space, Star Wars, Bad Piggies). Four of the have "with Friends" in their names. There's a smattering of big-name mobile titles—Plants Vs. Zombies, Fruit Ninja, Minecraft, SongPop, Plague Inc.—and after that it's all free."

Source:  Kotaku, 2nd January 2013