We may think he's making 15 million dollars but it's all speculation. The UFC and Conor loves to exaggerate numbers without real proof. To this day we don't even know PPV numbers because they are kept secret. Speaking of which, during a UFC conference for investor some of the PPV numbers were listed and they weren't as high as Dana White claimed. I think those numbers were for UFC 100 and 196.
Personally I believe age vs money comparison is kind of goofy. But if you compare the two, Mayweather vs Dela hoya earned 25 million dollars and 52(?) million in 2007. That's equivalent to around 29 million dollars in 2017 for Mayweather in one fight. And they both totaled 77 million dollars paid out in 2007. Did the UFC pay Nate Diaz or any of Conors opponents as much, percentage wise? I doubt it. The UFC is still shafting everyone.
Either way, as long as the UFC has a leash around Conor Mcgregors neck, he will never make as much as Mayweather.