The problem with ESPN's story is it completely lacked context; and it's obvious it is deliberate to make the UFC look bad.
Dave Metzer's Yahoo Sports article at least includes some context:
The reality is the top UFC fighters make very good money; lower tier fighters not so much.
A little perspective...
UFC revenue is estimated to be $350 - 450 million per year.
NHL revenue is listed as $2.7 billion per year. Hockey earns 6 times more money than the UFC every year.
10 years of current UFC revenue is $4.5 billion. The NFL makes more than double that in 1 year.
GSP is probably the UFC's highest paid fighter. He makes $4-5 million per fight; ~ $3-4 million is from the UFC excluding sponsorships. 2-3 fights per year is $6-12M.
The top NHL player earns $12M and the top NFL player earns $30M per year. So as a % of overall revenue GSP is doing better than the major sports.
A handful of boxers do better. Why? Simple...
Floyd Mayweather vs. Victor Ortiz grossed $78 million just on pay-per-view revenue.
If UFC 141 did 800,000 buys gross revenue would be ~ $36 million and the UFC only gets about half so $18 million. Heck...give 'em $25 million.
If you include gate revenue the discrepancy gets even larger.
The Mayweather-Ortiz gate was $9 million; Big UFC gates are in the $4-5 million range.
So $87 million to $30 million...that's why top boxers get paid more.
Lower and mid-tier UFC fighters actually do a bit better than boxing counterparts. No question though both don't make a lot.
Of course who sells more ppv's? GSP or Mike Pierce? 75 percent of UFC revenue is from ppv buys. The fighters bringing in the $$$ earn the $$$.