Well, If we look at other indy game makers like say, Fractal Games; They probably had immense benefit from selling their games on steam.
The first games they made were the Penumbra series witch was sold as boxed copies and (probably) as digital copies from their homepage. (They are now) It was published by Paradox Games.
There are no numbers what I can find of how many copies they sold of Penumbra + expansions, but when they were making their new game, Amnesia, they expected to sell some 20-40k copies.
Amnesia was a similar game and should have sold a similar number of units. But they did things different this time around. They put Amnesia up for sale on Steam. And well, in the first year Amnesia sold 400K copies!
Although probably not all of those extra sales are due to steam. Well, it still paint a pretty convincing picture.
Amnesia was priced to 15 euros/dollars but 75% of the copies were sold during sales so the price was marked down. It was still worth it in their opinion because the sales generated 50% of their revenue.
Eh, you people can read more about it here:
http://frictionalgames.blogspot.com/...l#comment-form