Walmart offers the LEGO Super Heroes Attack on Avengers Tower, model no. 76038, for $49.74. (Amazon and Kmart have it for the same price.) Pad your order over $50 to bag free shipping. (You can also opt for in-store pickup.) That's the best price we could find by $2, outside of the mention below. It includes 515 pieces and features five mini-figures and assorted accessories.
A close price: Target has it for $49.99 with free shipping.