Yeah but the new content in it completely worth 20$, so you kinda pay for the game again, but since GameStop has a 20% trade bonus towards Dark Arisen you can either trade it (which will cost about 25$) or give it to a friend who never had a chance to play this great game. Plus with your original save file you'll get extra armors (skins) and infinite ferrystones and something else I think. 
The expansion has a tons of new contents, fixed things from the original and a new area to explore, I've paid 20$ DLCs with way less content than this.
But they totally should have also release this as a separate DLC, they say that they couldn't or it would be to complex... it sounds weird, but since I know nothing at programming I can't really argue about that