Well, don't they say that almost half of PC sales are digital downloads? And pretty much all of that comes from Steam, so that number doesn't contradict my estimation at all.
But how do you know it sold 125K on Steam?
I think that the people that say half of PC sales are digital downloads are the ones like shio trying to justify the decline of the retail market by over-inflating digital sales. Digital sales aren't as strong as some would have you to believe them. They are increasing greatly, but I don't think it is at 50% yet. Maybe 30-40%.
Although Steam has a huge chunk of digital market share, its not quite as true for 3rd parties. The closest analogy one can find would be Nintendo vs. 3rd parties. Valve takes a huge chunk of revenue from its titles on Steam, which over-plays the market share of Steam. Although it probably has 60-70% of annual revenue each year, taking out the Valve titles, and its probably closer to 50%. So you have D2D, GamersGate and the others probably boosting borderlands to ~200k between all digital formats.
As for how I get the data:
www.fadellc.com
They have Steam data, so I looked it up
Back from the dead, I'm afraid.