It's tough to say. I don't know how much sponsors are paying the networks to advertise during fights for the non-PPV events. That's where the money would have to come from for a FOX or ESPN fight, and I'm not sure how much they are shelling out for Crawford vs. Benavidez or Spence vs. Ocampo. I do think these fighters would be boasting more about earning more if they were really getting more than the reported $2 - 3M per fight. If for no other reason it's used for leverage during negotiations.
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