What are the total sales figures for the game? Only Valve and Turtle Rock know for sure, but we can assume it's an ample number given how much Valve advertised Left 4 Dead on Steam and how well the Steam platform in general has been doing. On top of that, in the middle of last month there was a thirty-fold increase in Left 4 Dead sales during a brief half-price reduction on the game.
The Left 4 Dead DLC packages, which Valve promises to keep free, certainly help these figures as well. It would be nice, though, if they've cough up some actual numbers.