For mayweather this fight is money + perfect record vs more money.
He can make 300m and most likely point his way to a 50-0 record.
Or he can lose, make 300m, ruin his perfect record and then make even more in a rematch.
Win the rematch, it's now 1-1 so they go for a trilogy and he makes even more again.
So if he wants he can make 1BN plus from a trilogy or perfect record and 300m. Let's see which he prefers, record vs money.