Reads the bio: "This standalone expansion will be available at no additional cost until April 10, 2015. The next great automotive adventure begins with the Forza Horizon 2 Presents Fast & Furious standalone expansion for Xbox One! For the first time ever, the Forza experience of thrilling driving and amazing cars is combining with the unstoppable Fast & Furious movie franchise to create an original adventure filled with adrenaline-fueled automotive action."
Even after the free download period, the game will be available to play forever without restrictions. I cannot speak for the 360 version but the Xbox One version was about 15GB when I downloaded it.
Check out the game here, which will show $9.99 until you log in: Xbox Store