As 4A previously stated, there won't be any added narrative nor awkwardly tacked-on multiplayer. Instead, you'll gain access to "Ranger Mode," a game modifier that reduces ammo "dramatically" while increasing damage of all attacks, including yours and those of your enemies. In addition, "Ranger Hardcore" will remove the already-sparse HUD entirely, and completing it will likely unlock some of the new Achievements (for a total of 250 Gamerscore).
That's a fairly minimal addition -- a subtraction even -- for DLC, but you'll also have to weigh the benefits of the new Heavy Automatic Shotgun, which is being introduced alongside the Volt Drive, "Miller's signature gun." (Not even pens survived the nukes?)
4A and THQ aren't ready to sign off on a release date yet, but it's due on Steam and Xbox Live Marketplace "in the very near future." Pricing is currently the more delicate issue, even if the "Ranger Pack" is clearly aimed at the game's biggest proponents. And it's especially delicate if you're in the crowd that's really into multicolored kung-fu teenagers with attitude.