Steam aren't a monopoly by a long shot (which I consider a shame, personally!). Origin, Windows Store, hell even Desura. And they aren't lording any kind of monopoly over developers - use it or don't. Use it and tie it into our platform, get a discount. It's not a monopoly, nor a monopolistic action, until you don't have a choice.
And nobody knows what they're charging except those who can't say. As as the quote says, compared to OTHER digital distributors, it's about the same. Sorry, but half the people I know who play games (things like World of Warcraft, etc.) have never heard of Steam anyway. They certainly aren't a do-or-die outfit. Hell, Humble bundles make MILLIONS for their developers in some cases, for smaller indie outfits. If you can't make money on Steam, there are plenty of other places for such work who'll be glad to throw money at decent projects.
P.S. I am looking to publish a game at some point on Steam, only an indie thing, but I've seriously considered it since before Greenlight existed.
Personally, I'd sacrifice quite a hefty percentage just for the personal satisfaction of saying my game was on Steam rather than the alternates. But, then, I wouldn't let myself do it for next-to-nothing either. And if Steam charge more as a percentage but get a ton more exposure and sales, that's surely better? And there's NOTHING that says you can only release a game on Steam anyway.
Sorry, if you want to be on the bigger, more popular services, you have pay somewhere along the way. And NOBODY can say what they're paying. For all we know, they could be the cheapest place in town.