Apples and oranges.

1 thing I know for sure is that you're not going to become HW champion just by taking up boxing for a few months have 3 fights and then become HW champ. See Brock Lesnar as an example in MMA. You're not going to beat the Klit brothers or Haye with a few months of training.

Then the pay disparity is huge. Joe Louis was making 600k fighting Billy Conn in the 1940s which is more than what 99% of the stars in the UFC are making right now today.

And notice MMA can attract the washed up boxers like Mercer, Toney, Mayorga, no A-list or B-list boxer is going to fight for a few grand when they can make millions. Just take a look at Amir Khan (b-list boxer) who most people don't know in America and he's going to get a guarantee 1 million fighting Malinaggi on HBO.

My 2 cents.