A lot of the UFC's financials were made public as part of the lawsuit. That's how people know the fighters are only getting 18%. I tried to find the article that goes through it, but couldn't find it now. I think it was Bloody Elbow or MMA Fighting, but could be wrong. If memory serves, they include USADA as part of fighter pay, too. So really, it's only like 15-16% of revenue that actually gets paid out to the fighters.
Bellator, ONE, PFL, etc. can't afford to pay the same as the UFC. They will strategically pick and choose some big names to sign away from the UFC, but for most fighters their best bet is to sign with the UFC...but that doesn't mean they aren't underpaid. That's like saying NFL players should be making $100k's instead of $1M's because hey...at least it's more than the CFL can pay.
I don't know what the split should be exactly, but look around. All the other sports leagues are around 50/50. When I looked this up before I think golf or tennis was the lowest besides the UFC and the athletes were still getting like 35% of revenue (edit: golfers and tennis players are also much more free to get their own sponsorships than UFC fighters).
It's a 'free market' (although we'll see how free the courts decide it really is) and ultimately it's on the fighters to do something to change it (i.e. unionize, policy changes). Until then, it is what it is, but the fighters are underpaid and are right to demand more. Most of the time it's not going to work, though, because they will just move on to the next guy in line. That's probably going to happen with Francis, too, but it's a unique situation because he could at least get out of his contract.