I have a former student who promotes Fearless Freestyle Fighting events in NE Indiana and has promoted them all over Ohio, Indiana and Michigan at one time. She once told me that by the time everything is said and done it cost her around $8,000 each time she threw a small event at a local place and around $20,000 to throw a major venue. Of course it depends on the place rented, how much marketing you do and if you pay big name fighters to show up and sign t-shirts and what not. Her biggest complaint was the cost of the insurance which was quite often around $2,000 for a single event with a 10 million liability limit.
She needed that insurance one time because she got sued...but she ended up winning the case because the fighter that sued knew well enough that it was possible they would get seriously injured (he broke his arm in Kimura).