How much do you have to pay to be a ref?
That's a complicated question, and this answer will be long! I mean, you can say right now, "Hey, I'm an MMA referee!" and in a lot of places where MMA isn't regulated or sanctioned, you are now just as official a referee as Jason Herzog. That doesn't mean anyone is going to let you referee their fight, though.
In a practical sense, if you want to work as a referee, you need to be certified by the jurisdiction where you work. For example, if I want to work a UFC event here in Texas, I'd need to be licensed by the Texas Department of Licensing and Regulation (TLDR). In Las Vegas, it would be the Nevada State Athletic Commission (NSAC); in Los Angeles, the CSAC. There is a licensing fee which varies from state to state and is usually pretty small.
However, in many states you need to be trained before you can be licensed, and even in the states where that is not required, nobody is going to hire you to referee an event if you can't prove you know how to do the job. The course I just took was $125, but that is the bare, absolute minimum you need even to get the process started. For example: false modesty aside, I know my shit
really well; I aced the test easily and would probably have passed even if I took it before the class. And even with that being the case, I would never
dream of trying to get licensed as a referee or judge until I had done at least 100 fights as an inspector, with the idea of shadowing/apprenticing from the judges and referees as I did so. And once I was licensed, I wouldn't dream of trying to referee a fight solo until I had spent enough time working with a good referee that he felt I was ready to go.
I think poor officiating is a serious, chronic problem within mixed martial arts—and every other combat sport I know of—even if I think the overall quality of referees and judges is actually pretty high. Since it's a known problem, however, I wouldn't throw my hat into the ring, so to speak, if I was worried I might make it worse.