Valve's "here, have 75% off" works because it is seventy-five percent OFF, not because it makes software inexpensive. If your software was inexpensive all of the time, it wouldn't help you.
Of course, it should also be noted that 50 bucks was way too high of a price for L4D in the first place. Valve was selling Team Fortress 2 for 20 bucks at the time, if I recall correctly (and it was 30 at launch). So I feel they got the boon of the sale, and also the boon of the "finally, a reasonable price" effect.
Anyway, it's the clothes stores that have neverending sales of their highly interchangable product. I don't think that Waldenbooks would see the same profitability curve as Valve by rotating sales throughout their stock though, as their product requires shelf space and falls within a much smaller price range.
As for L4D's initial price, Valve was smart to set it relatively high, as a multiplayer game like that leverages peer pressure to motivate sales.