Eh, that always tends to happen with these series. By episode 3 or 4 release the seasons always get stupid cheap on discount. Sales numbers don't stay flat afterall throughout the run and this helps win people in as media attention dies off after episode 1 releases.
I bought Batman for 15$ a month after the last episode came out(a 40% discount too), and I don't think it's had a significant drop since, but I'm not sure. From what I can tell, it was the same with Minecraft, a 50% drop 2 months after the last episode released. This is only on episode 3, which is coming next week, not even out yet, and it's cheaper than Batman and Minecraft. That's not good.
Maybe it's just that all of their games do poorly, except Batman, Minecraft, and Walking Dead.