Friggen Sonnen, only claiming $50K? I'm sure that number is for IRS consumption only ;-)
Just kidding .... I think?
Actually I suspect your joke is close to reality. I know a lot of companies nowadays that just do under the table cash transactions, completely off the books, to pay their employees and keep the tax burden low for both themselves and the employees.
There is no way UFC level fighters could be fighting on $18000 a year minus training expenses (that is the UFC entry level purse I think, $6000, and that level of fighter probably doesn't have any endorsements either). You can make more than that working at McDonalds, imo there's just no way the top .1% of fighters in the world (which is what you are if the UFC signs you) actually take that home, future prospects or no future prospects.
Come to think of it, I've always suspected the same in boxing.