Apparently the belt Mayweather Jr. cares about is the WBC belt.

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WBA: Appendix D
For world championship contests (in US $):
1) PROMOTER’S BOUT FEE
TOTAL PURSE OF THE BOXERS PROMOTION BOUT FEE (US$)
Up to $ 250.000 $ 6,000
FROM $ 250.001 TO $ 500.000 $ 8,000
FROM $ 500.001 TO $ 1.000.000 $ 10,000
FROM $ 1.000.001 TO $ 4.000.000 $ 15,000
FROM $ 4.000.001 TO $ 7.000.000 $ 20,000
FROM $ 7.000.001 TO $ 10.000.000 $ 25,000
FROM $ 10.000.001 Ahead $ 30,000

2) Sanctioning Fee Percentage of Purse Minimum Maximum
Champion 3% of purse $ 3,500 $ 150,000
Challenger 3% of purse $ 1,500 $ 150,000

If both fighters are recognized as champions, each is subject to the champion’s fee. If both fighters are recognized as challengers, each is subject to the challenger’s fee.