From what I've read, there have been several cases (at least with the UFC) in which a fighter has been left without an opponent as late as fight week & they've been paid/compensated for their time. I wouldn't imagine it would be close to their full fight purse just more something that covers the fighters costs. If it's anything further out from fight week the UFC tries to re-book the fighter at the next available card which is usually only a week or 2 away.
Seems as though across MMA, Boxing & Kickboxing at least, the promoter is under absolutely no obligation to pay anything out at all. Insurance seems to be limited to only covering medical costs of the fighter pulling out & not a lose of fight purse or time invested to either fighter.
My understanding if a fight falls apart, the UFC is not obligated to pay fighters their show money. However, in the past, UFC will generally pay fighters show money if a fight falls out at the last second because crap happens and the opponents are still showing up, ready to fight, and on weight, so they get something for their time
However, Demetrious Johnson was there, he didn't have to make weight and stopped cutting weight. There was no fight. So from a promoter perspective, why should they pay someone for work they did not do?
Johnson's payout for his fight with Wilson Reis was $350K. He also got a performance bonus of $50,000 and a Reebok bonus of $40K. So his disclosed payout totaled $440K. That's not necessarily chump change for MMA. But that's because he made it to the fight, went into the cage and fight.
I think there is insurance for fighters to cover training injures and fight injuries, but that's it.
A fighter's job is to fight. So if they don't fight, why should they get paid? Ariel Helwani claims that the money is allocated anyway, so just pay him. Well no, that's not how business works. Johnson lost his opponent and the UFC didn't get the fight they were marketing. That scramble made them lose money if anything. So from a business perspective, it makes no sense for a business owner to drop $350K on Demetrious Johnson for a fight he didn't take part in.