I want to buy the new chapter pack but don't know if I should wait to see if it goes down in price over Christmas
I would imagine there would be a small Christmas sale coming up
The game is already on sale on Steam and Xbox, and according to the latest dev stream it should go on sale on PS4 some time this week if it isn't already. The sale will last until early January.
However, the new DLC isn't on sale on Steam, presumably because it just came out, so it probably won't be on sale on consoles either.
(Edit: The game is now on sale on Xbox as well as Steam)
There's a Xbox community sale at the moment, so i'm sure PC and PS4 will get one soon after.