Luckily, I got Serious Sam TFE and TSE HD editions 75% off back in May 2011. They were also put 90% off on Steam's holiday sale while BFE was 50% off for being "new" and not as crowded as Skyrim or Modern Warfare 3.
I'm not sure who chooses the sales, since Duke Nukem (will) Forever (be known as a letdown) got lots of sales. If it's Croteam who made the sales, then that sure is nice. If Valve's Steam team decided the discounts, then it might be about the Metascore or something.