Here's the dirty little secret of discount pricing: it only works as long as people think it's a discount. As soon as they leave the page they see it on, it isn't a discount anymore, it is just The Price.
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.
The other motivation is that sales end. Once someone thinks that they could save X*3/4 dollars if they bought it now the question has become when they're going to buy it, not if.
I always cock an eyebrow when I'm at the mall (usually dragged along) and I look around and see that everything has the same 25% off tag.
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.
Valve versus mall shopping is an interesting comparison in that it has shows some strong underlying differences. Anecdotally, most mall shops are clothing, an area where a lot of items contain the same basic function of nudity prevention and weather protection, plus some fashion value of relative merit. Valve's products are more akin to the one or two bookstores in the mall, where the basic function of the product is entertainment where the entertainment value of the product is enhanced by differences between individual products. Similarities do occur within genre, but differentiation is still necessary for success.
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.
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.