80% of anything Google sells and 65% of anything additional the network sells.
Tarmack is right, when ads run against your videos, the MCN takes 20% of your cut (after YT takes their share) from adds sold by Google/YT. However, the MCN ("Network") also has a sales team trying to sell ads for their videos - when they do that, they take 35% of your cut (after Google's share) instead.
However, as I said in my post on page 2, the important part isn't how much they're taking, it's what are they offering you in exchange. If I told you "hey, every time you have a dollar, give me a dime for doing nothing in return." you'd (hopefully) say no, because even though it's only 10% of your dollar, I didn't do anything to deserve it.
However, if I said "I'm going to work my butt off and really help you make a bunch of dollars, but in exchange I'm going to take 40 cents of each one for myself" it would be a different scenario, because your keeping 60% of a lot of dollars you wouldn't have without me.
So before you worry about what they're taking, figure out what they're offering. Only then can you decide if the split they require is worth it.