Question about the S.T.A.L.K.E.R series .

Not sure about your first question. There is a site that tells the frequency of games being on sale on Steam, (I am sure someone will post it) but I don't know it. I think STALKER 2 got cancelled to answer your 2nd question. After buying these three games, be prepared to download a hella lot of mods, and be prepared for some glitches. Otherwise, STALKER is a good series of games.

You might find them for $5 or even $2.50 each in a sale, but definitely pick them up when they are - some of the most deeply atmospheric games out there. Can't give sale predictions, though a Stalker sale on Valentines Day would be hilarious. Unfortunately there will never be a Stalker 2, but much of its original team are making Survarium which will be kind of an online S.T.A.L.K.E.R. co-op experience with PvP.