When the cost of the season pass for Batman: Arkham Knight was announced, many people were taken back by the $40 price tag. That's close to the price of the game itself. Over the weekend, the Batman Arkham Facebook page gave some details on what you can expect for DLC in the months after the game's released.
The biggie is the ability to play as Batgirl in a prequel story. Batgirl's originally Barbara Gordon, daughter of Commissioner Gordon. In Arkham Knight, she's known as Oracle using technology to help out Batman after The Joker paralyzed her and ended her stint as Batgirl. With the DLC, you'll get to experience Batgirl before her unfortunate turn of events leading to her being confined to a wheelchair.
The rest of the DLC announced are:
Now, whether this is worth $40 is up for debate and there's probably DLC not announced yet. Being able to play a prequel story as Batgirl is a pretty good start though.