Cliff Harris, indie developer famous for games such as Gratuitous Space Battles, the Kudos games and the Democracy series, has revealed via his Twitter account that Democracy 3 managed to make 64% of its Steam revenue at full price:
19% of the Steam revenue was for when the game was at 50% off and 16% at 66% off. This tends to fly straight in the face of what many other articles by indie developers suggest about the effectiveness of Steam sales in driving revenue such as this one, this one and even this one.
Cliff hasn't provided any volume figures (yet) and you'd obviously expect at least high volumes during sales, but in the end a developer that is looking to make dough probably only cares about the revenue, unless of course you're planning to release heaps of DLC for your game... then having MOAR copies of your game out there means MOAR potential DLC purchases...
Also, the type of game, whether the game is a sequel or not, the developer's history and starting price, probably all have a part to play in determining how effective a Steam sale truly is. When you're comparing things that are a weird mixture of technology and art (i.e. computer games), you're not really comparing apples for apples (unless they happen to be Macs, huehuehuehuehue).