The prize pool for the next Dota 2 Championships has already passed a staggering $4 million dollars. The International, an annual tournament publicised most recently with the release of Free to Play (which focussed on players in the inaugural competition), is being partially crowdfunded.
An initial lump sum of $600k was put up by Valve, and this figure is being supplemented by sales of the Compendium ($2.50 is going over from every $10 paid - that's well over 1 million sales since last week). There's stretch goals in place if the fund exceeds $6 million, and if it does, it'll certainly make for an even more fiercely competitive event.