Lest anyone thinks that this is not big business, the largest prize pool in gaming is the Dota International (held once a year).

It's currently sitting at > $12m (split between 5 players on a team, their manager, and who knows who else). It will probably rise to over $15m by the time the tournament is on.

Of interest is that the prize pool comes from the "normal" players playing the game - you buy in game items, and 25% of the revenue from that goes to the prize pool for the tournament. The items themselves are completely optional and cosmetic only. And, of course, valve takes the other 75%.