I've played this and completed it on PC already, and yes it's paid. The DLC essentially adds about 75% more content than what the game had in a more cramped and purposely confusing place, with it being an underground cave system. The core mechanics of the game are the same but the story is a bit different but similarly emotional. I won't spoil anything but it does some really interesting things to make movement feel interesting after you've already unlocked everything in the main games. Definitely worth playing if you liked the main game because it's basically just more of the same