None of the money goes to Valve from games sold over Steam, unless of course they are Valve games.
Actually, some of the money DOES go to Valve for games sold over the Steam Digital Store. However, Valve does claim you don't have to pay for the use of SteamWorks technologies (and Steam DISTRIBUTION, which isn't the same as sales!)
(My reasoning that money changes hands for games sold over Steam is due to an article from 2005, and Valve's usage of the word "free" when describing SteamWorks, and nothing but a "Contact Us" link when describing end-to-end Steam publishing. That smacks of "costs so much we don't want to frighten you off too early")
Now if anyone here actually has a contract with Valve that can correct... no wait, the NDA would forbid them from answering.