If I had a dollar for every time that happened to me either through Steam or GOG or PS3/4 games, I'd buy a second GTX 1080 Ti. But I will say this: if you buy a game that is more than say a year old like you did (two and a half to be exact in RB6), expect it to only drop in price over time. So while you paid $40 for it and it's now $20 for a short time, that's just the way things go. You had no way to know it was going on sale on Steam.
These things are hit or miss and a lot of it depends on how popular the game is still selling a year or longer after release. Steam felt it was still worth charging $40 for until this weekend. My guess is that it won't be long before the regular price falls to $30. If you want a game, buy it. No sense...