I have a couple friends who have books on there. They get paid $0.0045 per page read. So basically if a large amount of subscribers read a very large amount of pages (which if you have Kindle Unlimited it is very likely) they the revenue to authors exceeds revenue to Amazon. Somebody smarter than me did the math that if a subscriber reads 164 books in a year (I'm already on book 15 for this month) at an average of 400 pages per book, the subscriber for a year pays $143.88 then at that rate of reading the subscriber could cost Amazon $151.32 in losses.
That's not how it works. It's more like a parimutuel pool.
For those unfamiliar with the parimutuel theft machine; a racetrack takes all the money they take in and rakes off 17% (!) then distributes what's left to the winning bettors. The odds are set by the amount of money bet on each horse. (Given the number of horses and the number of ways one can bet on them, it is obviously a complicated calculation, but that's the basic idea.) No matter what, though, the track gets their 17%, or whatever the legal limit of their take is. (By way of comparison, the house take on the very lucrative game of roulette is about 5% for American-style roulette with 0 and 00 on the wheel. No wonder people go quickly broke playing the ponies!)
Amazon puts all the money paid for Kindle Unlimited into a pool each month, then divides up the money based on your book's number-of-pages-read as a percentage of all pages read. Just like the racetrack, Amazon keeps their take, no matter what. They even publish the dollar amount of the pool each month.
It is a lousy deal, in part because some author/publishers pay overseas "readers" to "read" their books. I imagine some script accomplishes this since it would be hard to make a profit by manually flipping the pages on a bunch of Kindles. That dilutes the pool for legit author/publishers. I only have one of my books on Unlimited, and that's because it is basically a promo piece that I'd happily give away if Amazon would let me.