We only got the announcement yesterday
and it appears that Scorpion is already available as DLC in Injustice: Gods Among Us
There is a catch however. I am assuming Scorpion might have been released early because he is currently only available to those who have the seasons pass. He isn't up for those who don't have it yet. No word yet on when he will be available for everyone.
There are however three other sets of DLC packs available for anyone to purchase. The first is the compatibility pack for Scorpion which allows players who don't have the DLC online to play against him. It also features a free Containment Suit Doomsday skin.
The second set is the Blackest Night skin pack which contains three skins for Superman, Flash and Doomsday. This pack is available for 240 Microsoft Points.
The third set is the Red Son 2 pack which contains three skins for Batman, Deathstroke and Green Lantern. This pack will also be available for 240 Microsoft Points.