2-63. That's if you don't know anything, where the chance of winning is 50% each game. To put this into perspective, that's about 250 times worse than winning back-to-back Powerball lotteries. Let's say you predict games correctly about 74% of the time, then the odds go up to a single Powerball.
In other words, don't be tempted by the lead that you will win $1MM if you get a perfect bracket. The time value of entering is well below the minimum wage.
[for non-US readers, I'm referring to the annual college basketball, single elimination basketball tournament with 64 teams]