Valve's put out life-time retail sales figures for the past decade, as reported by Gamasutra.
The numbers were published in the latest issue of Game Informer in the US, and only include retail sales - there are no Steam stats in here.
All the info below. This should give a clear indication of why Valve probably isn't so worried about paying the mortgage.
- Half-Life - 9.3 million
- Half-Life: Opposing Force - 1.1 million
- Half-Life: Blue Shift - 800,000
- Half-Life 2 - 6.5 million
- Half-Life 2: Episode One - 1.4 million
- The Orange Box - 3 million
- Counter-Strike - 4.2 million
- Counter-Strike: Condition Zero - 2.9 million
- Counter-Strike: Source - 2.1 million
- Counter-Strike (Xbox) - 1.5 million
- Left 4 Dead (projected) - 3.6 - 3.9 million
Combined retail sales - 32.8 million (36.4 - 36.7 million if the Left 4 Dead projection's included)