30% is the standard retail cut if I’m right. So if you sell your game at Walmart, they take 30% too.
Edit: There is actually a way to bypass Steam’s cut - provide by Valve strangely enough - if I’m right. As developer you can mint as many API keys for your games as you like and sell those through other means. Your customers will still download and play through Steam but Steam gets nothing - i.e. you use their infrastructure for free; of course they would get their cut from copies sold via their store.