Steam Spy tracked the recent Steam Winter Sale, and as you'd expect, its creator Sergey Galyonkin wrote up a blog post with interesting takeaways. The most salient for developers seems to be this: A discount in the 50 to 79 percent range seems to be the most profitable.
That's an important question to answer, since it's often hard to know how much (or how little) to discount your game -- balancing user expectations versus what's safe to do in terms of profits is a tough question.
To illustrate his point, Galyonkin dug into the data and pulled out the revenue figures for the #10 best-selling game at each discount percentage. The swath running from 50 to 79 percent was most profitable. We've reproduced his graph below.
And bear in mind, as always, that there's a significant margin of error on Galyonkin's data. However, multiple developers have vouched for its accuracy to within that tolerance.