Valve takes about 30% of any store purchase made on Steam (last I checked they weren't allowing anyone with a contract to publish the actual number, that may have changed). It's fairly common for a store to sell stuff for more than they pay themselves, it's part of how they can pay their employees.
And it's not like it's pure profit for them after paying wages. They still have to provide a download for any required files, and not just as a one-time download either. If you get an entirely new computer, you can just download all the games in your Steam library again. Which means a constant expense to keep the servers running and making sure that there are backups of every game that has ever been available on the service. And bandwidth costs money as well, especially for the largest online games platform. And depending on the service provider they use for it, each credit card transaction will cost them a bit of money as well.
Then there's the various community stuff like the Steam forums, family library sharing, workshop, friends list and chat. All of which costs money to develop, maintain, and refine while not directly contributing any to their income. The market place does generate some cash flow for them, but that is once again due to Valve taking a cut on every single transaction (you can even see how much whenever you buy/sell something on the market place).
Make no mistake, Valve are making quite a bit of profit from Steam. But if they didn't, they probably would not expand or push the extra community related stuff as much as they do.
