Amazon offers the K'Nex Robo Smash Building Set, model no. 13243, for $7.99. (Walmart has it for the same price, via in-store pickup.) Plus, Prime members receive free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.18.) That's tied with our mention from over a year ago and the lowest total price we could find by $13, outside the price below. This 156-piece set requires two AA batteries (not included), and once it's constructed it hops around like a primate.

A close price: Target offers it for $8.09 with free shipping.