Best I could find is that they’ll wave fees on the first $15,000 in sales, which is pretty damned low. Then it’s 30% until you get in the millions of dollars, and then they’ll cut it to 25%.

I’m not sure about “vast majority”, but I’m sure that lots of devs are making no money purely because they sell basically no copies. But if a game has even a moderate success, it looks like Steam’s cut will be 30%