Amazon offers Prime member the King of Tokyo Power Up Expansion Pack for $10.83 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping adds $4.55.) That's the lowest total price we could find by $4.
This expansion pack requires the King of Tokyo base game for activation. Orders placed by December 17 are guaranteed to arrive by Christmas; Prime members can order by December 21 for guaranteed arrival.