With orders of $25 or more, Amazon offers Prime members the Imagine by Rubie's Kids' The Amazing Spider-Man 2 Muscle Chest Shirt and Mask Costume Set for $6.68 with free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's the lowest total price we could find by $20. It comes in one size, to fit most children up to 48" tall with up to a 26" waist.
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.