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Here’s how much Valve pays its staff — and how few people it employs
(www.theverge.com)
This is a most excellent place for technology news and articles.
They can even list there and sell Steam keys on their website and not pay any of that to Valve, with the only stipulation that Steam keys cannot be sold for less than on Steam itself.
So basically:
You only pay the 30% cut for sales made through Steam.
That's incredibly reasonable.