It's only a good bet if you win, especially when $6 million is apparently involved. Per the NY Daily News:

The fighter is reportedly betting $5.9 million on Monday night's Game 7 between the Pacers and the Heat, going with Miami, a seven-point favorite against Indiana.

According to Vegas Gambling Steam, which reported the news of the big bet on Twitter (@Pregame_Steam), Mayweather had to spread his wagers across nine sportsbooks.

We can safely say we've never had a bet so large that one casino could not hold all the action.

But hey, with a big Heat victory we can't say it was a bad bet.