Super Data Research who specialise in collecting data with regards to the sale of digital products has compiled a list of the top free-to-play earners worldwide in 2013. Don't be surprised if you haven't heard of the number one game - Crossfire by Korean outfit SmileGate. The game is availble across the world, but is particularly popular in Asia. As you may expect when the words free-to-play, shooter and Asia are combined it is sorted into the pay-to-win category. Players wanted improved weapons to the tune of $957 million in Crossfire in 2013 according to Super Data Research.
A distant second is League of Legends at $624 million with Nexon's Dungeon Fighter Online in third. Wargaming's World of Tanks took an impressive fourth (who dares claim tanks are niche now?) with $372 million. It would seem that Blizzard's venture into micro-transaction with World of Warcraft is paying off as it managed seventh with $213 million. The fact that EA squeezed $139 million out of Star Wars: The Old Republic shows that perhaps it's not quite as big a write off as previously assumed.
As you may have guessed these are researched estimates and not actual numbers, but they're a good indication of the money there is to be made in this space regardless.