CDKeys offers downloads of Halo 5: Guardians Digital Deluxe Edition for Xbox One for $18.99. That's $26 under our March mention (which was for XBL Gold members) and the lowest price we've seen. (It's the best deal today by $51.) It includes the full base game, Warzone REQ bundle, and Halo: The Fall of Reach animated series.
Note: New customers can provide an email address on the CDKeys Facebook page to get a unique coupon code that cuts an extra 5% off