If you have Amazon Prime, you can have Twitch Prime. And if you have Twitch Prime, you can get a $15 Amazon credit when you buy a copy of GTA V (PS4 | Xbox One) for $15, meaning it’s basically free.
Just make sure you’ve created a Twitch Prime account before making your purchase, and double check that the game is being sold by Amazon.com or Amazon Digital Services LLC directly, or else you won’t get the credit (which expires at the end of January).
Full terms and conditions here.