I'm asking because they have to pay for the network operation. Other than that, I'm thinking it's pure profits. No retailer, distributor, etc. If this is the case, I'll buy through Steam so Valve gets more cash.
You guy's arent counting in the fact that a Steam-only distribution would alienate quite a few HL2 fans... Teens without creditcards. Then there are the people who still feel insecure about online transactions. If Valve released HL2 on Steam alone, it would be incredibly stupid.
Usually, the publisher is only really needed to pay for the salaries of the developers, boxed games and the marketing of the final product (marketing = all advertising + having retail channels lined up for distribution).
In this case, vALVE paid the salaries using the money from the first game/mods, and the game really does not need advertising [:grin:]. So the publisher really cannot charge them a large percentage of game's profits. Instead of 30-50%, it may be, like, 10%.
When distributing through Steam, there *is* the cost of bandwidth and servers throughout the world. But these costs shall be reduced by having a peer-to-peer downloading client.