The costume can be yours tomorrow from the PlayStation Store.
Ordinarily you'd need to complete the game in order to access the skin, but Rocksteady's Sefton Hill used the free DLC's announcement to share a code that allows you to use any downloadable skin in the campaign without having to beat it first. At the main menu, after selecting your save, you'll need to press: left, left, down, down, left, left, right, up, up, down. That'll make all of the game's costumes available regardless of your progress throughout the campaign. How old school is that?
And if you absolutely must pay money for something, Rocksteady's got your back for that too. It's announced the "Arkham Bundle" which collates the Nightwing Bundle Pack, Robin Bundle Pack, and the Arkham City Skins Pack for £11.99. The previously announced Challenge Map Pack is also due out this week for £2.39, granting you access to the new Batcave map, the Joker's Carnival, and the Iceberg Lounge.
There's never been a better time to pop that copy of Batman: Arkham City back inside your console.