Mayweather at no point officially offered 50/50. Manny wanted 50/50.
Mayweather offered Pacquiao 40 million, where as he was set to earn over 100.
With drug testing, it has all been exposed recently. I can't remember the names, but Pacquiao has recently accepted Olympic Style Drug Testing (and looked the exact same, ending PED rumors) while Mayweather refuses to go away from his specific testers. Everyone uses one like NADT or whatever the name is, and Mayweather refuses to leave USADT or whatever. Mayweather makes 100million a fight, he could easily be paying them off, so you can't blame anyone for refusing it.
Thanks to Ape for the sig.