I did some searching around and it's true. There is no set percentage. Uber takes whatever they want and tosses drivers the scraps. I could have sworn they said 25% back when I first started this crap.
Lyft does the same thing and probably started it, just uber was straight forward about it first and Lyft recently allowed drivers to see what riders actually paid also.
It wouldn't surprise me if you never check the fare details, as they track all the buttons you push that they take more from people like you then people who never check. At least that's what this evil guy would do if I was in charge of uber lol