This week the Hearthstone team at Blizzard slipped a competitive tournament with a $10 buy-in into the free-to-play card battler via its week-long "Heroic Tavern Brawl" event.
What's interesting here is that while there have long been Hearthstone tournaments, they've typically taken place outside the game itself -- in a dormitory, a stadium, or the like. Now there's an in-game tournament anyone can pay $10 (or the equivalent amount of in-game currency) to compete in, though prizes are limited to in-game rewards rather than real money.
Competitors play against each other, and winners progress to their next match while losers exit the Brawl. The more matches players win (up to a maximum of 12), the better their rewards -- based on Blizzard's chart of winnings, it seems like a player would need to win at least 4-6 matches to recoup the cost of entering the event.
And while the Tavern Brawl (and similar modes in other Blizzard games) has long been used by the Hearthstone team to experiment with new ways of playing the game (in previous weeks Brawl players have been asked to play Hearthstone cooperatively against AI, for example, or play against each other with randomized decks), now it seems Blizzard is using it to experiment with new ways of monetizing as well.
Incidentally, earlier this year Gamasutra highlighted Hearthstone's Tavern Brawls earlier this year as a particularly interesting and effective use of an in-game event system.