Amazon offers its Prime members Darksiders II: Deathinitive Edition for Xbox One for $19.68 or PlayStation 4 for $19.99 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's the lowest total price we could find by $2.

Of note, Gamestop also has it for PlayStation 4 for $19.99 with in-store pickup.

Update: The Xbox One version has risen slightly to $20.72.