Special Supply via Amazon offers Prime members the Mega Bloks Teenage Mutant Ninja Turtles Sewer Lair, model no. DMX12, for $14.38 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.59.) Except for the mention below, that's the lowest total price we could find by $4. It includes turtle lair, Raphael, Kraang, and mouser.
Note: Walmart has it for $14.99 with free in-store pickup.