You are vastly overestimating how much is made on ppv points. Below is a report on this very topic, giving a breakdown of how much at each step a fighter gets.
Essentially, a 600,000 buy card would net $750,000 in ppv points.
Also, other info released as part of the antitrust lawsuit suggest normally only the Champion is on ppv points, not just main carders, and almost definitely not co-main, unless it's also a championship fight.
Note, the UFC do not stick rigidly to the outline below, but in general it's how it works