Just noticed that Fifa 15 Deluxe Edition is currently on sale on the Xbox One store. If you visit the web site you will need to log in to see the price otherwise go through the Xbox One.
This is perhaps not the cheapest it has been, but as far as I can tell it is the cheapest at this point in time. Also, as this is a deluxe edition which can't be bought in store, it may be better value offering the additional content below.
Bundle Description:Receive the full game including 2200 FIFA Points to redeem FIFA 15 Ultimate Team Packs, a Leo Messi FUT Loan player for 5 games, and a bundle of popular EA SPORTS Football Club items. DELUXE EDITION CONTENT AVAILABLE ONLY FOR NEW PURCHASES AND EXPIRES JANUARY 19, 2015. 2200 FIFA POINTS BONUS CONTENT UNLOCKS A MINIMUM OF 22 GOLD PACKS.