Amazon offers Prime members the LEGO Creator Cargo Heliplane, model no. 31029, for $8.59 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.13.) That's the lowest total price we could find by $11, outside of the mentions below. This 132-piece set can be used to built a boat, plane, or helicopter.
A close price: Kmart and Barnes & Noble both charge a buck more with in-store pickup.