Well, it's a billion dollars in ad revenue, but keep in mind, the NCAA pays for airfare, hotels, etc, for every team in the tournament. That's going to add up. They have to pay for every single expense at every single regional site, from paying the officials, to the people running the scoreboard, etc. Literally, they pay for every single aspect of the tournament out of whatever revenue they bring in.
Obviously there's going to be a lot of money left over, but I don't think people realize how much they have to spend on championships every year. It's absurd, especially when you consider they're paying for all of these expenses at D1, D2, and D3 levels, across all sports, and the D3 swimming and diving championship sure as hell isn't bringing in any revenue to offset the hotel and airfare of the teams and individual swimmers competing, for example.