It's something like $1000 per event plus expenses (Travel, Hotel, Etc.). Plus they are not UFC referees, they are the state's athletic commissions refs, and they are assigned to the matches that are in the state, regardless of the promotion.
Here's a great article on Mario Yamasaki:
http://www.washingtonpost.com/wp-dyn/content/article/2007/07/10/AR2007071001618.html