What is SteamDB?
SteamDB [steamdb.info] is a website that keeps track of the prices of games sold on Steam. With it, you can see what was the highest discount a game on Steam ever had. For example, if during a sale a game is 20% off, but SteamDB says the lowest discount it has ever had was 50%, it's possible (although there is no guarantee) that in the future the game will be on a sale with 50% discount again, so if you wait, you may save money.