Boxing pays way more because it has been around for over 50 years. How much do you think boxers made in the first 15 years of the sports existence?
LOL
and like the UFC, it is simply not that simple.
for most of the history of boxing there was little revenue to be made in boxing. basically, ticket sales.
some time in the 50's a tad bit more was made from a TV contract (you know, back when there were 3 channels and few advertisers) or - in the 70's or so - a "closed circuit" style. and then, of course, HBO & Showtime & PPV. and THAT is when pay exploded.
until the late 70's when cable TV picked up and tv revenue's were capable of paying for the sporting event, boxers were paid from revenue generated - ticket prices. and yes, Sugar Ray Robinson fights demanded higher ticket prices so he got paid more than other fighters of the time, but he got paid peanuts compared to anyone in the 80's.
Ali & Foreman got paid a lot of $ because a despot paid them a lot of money to have that fight in his country.
boxing pay 20 years into boxing and MMA fighter pay 20 years into MMA really have little in common.
today the highest paid boxers are those that use their own production company. they removed the middle man. DLH started this i think. many have followed.
some day a fighter will be popular enough that he will leave the UFC and try his own hand (e.g. remove the UFC middle man). Couture & BJ tried & failed. Lindland and many other fighters have started local orgs, but it was just too much damn work for too high risk & too little profit.
it would have been interesting if Couture & Fedor could have pulled it off.