Amazon offers its Prime members the LEGO Ninjago Kai's Fire Mech Play Set, model no. 70500, for $7.57 with free shipping. (Non-members can spend $25 or more to get free shipping; otherwise it adds $6.04.) That's the lowest total price we could find by $3. This set features 102 pieces, including two minifigures with weapons.
Orders placed via Prime by December 22 are guaranteed to arrive in time for Christmas. (The deadline for standard shipping is December 19.)