When UFC was brought for 2 mill they STILL were makeing a loss, they just seen it as a future money maker, which it now is.
Imagine training for 8 to 12 weeks, 10 hours a day, 7 days a week, to make $4,000!!!
A lot of fighters are also trainers and get paid for that as a more "regular" job. Also they usually make more than 4k and its the lure of makeing a million a fight if they are good enough.
In the end they get 4k to get beat up. If they win they get another match and get 8k etc till their on millions if their good enough. Why would you pay someone $100,000 to get beat up, hell id get in the ring for that. And they can fight up to 8+ times a year (the lower paid ones) so that can be around 30k easy, which for people who's only skill may be beating the crap out of people isnt bad for someone who's not even good at it.