Amazon offers Prime members the K'NEX Robo-Sting Building Set
, model no. 13244, for $6.39
with free shipping
. (Non-members can spend $35 to qualify for free shipping; otherwise, shipping costs $6.18.) That's the lowest total price we could find by $3. This 157-piece set has motorized features and can combine with other Robo Creatures sets. It requires two AA batteries (not included).