Amazon offers Prime members the Star Wars Kids' Darth Vader and Clone Trooper Costume Set for $18.76 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.83.) That's the lowest price we could find by $20. It includes a Darth Vader jumpsuit, cape, mask and light saber, and a clone trooper jumpsuit and mask. It is designed to fit most children 50" to 54" tall with a 27" to 30" waist.

Update: The price has increased to $21.38.