Last weekend, Valve decided to do an experiment with Left 4 Dead. Last weekend's sale resulted in a 3000% increase over relatively flat numbers. They sold more last weekend than when they launched the game. WOW. That is unheard of in this industry. They beat their launch sales. Also, they snagged a 1600% increase in new customers to Steam over the baseline.
Worried retailers, fear not. The weekend sale didn't canabalize sales from retail. In fact, they remained constant. Well, constant isn't a 3000% increase, but it's still pretty good, right?
Looking at a third-party game, they saw increases of 36,000% with a weekend sale. Oh. Em. Gee. Okay, Gabe is starting to convince me that PC at retail is going to die very soon.
During the Holiday sales:
10% sale = 35% increase in sales (real dollars, not units shipped)
25% sale = 245% increase in sales
50% sale = 320% increase in sales
75% sale = 1470% increase in sales
At 75% off, they are making 15% more money than they were at full price.