Amazon offers Prime members the Kids' Batman Action Suit Costume
in Blue (pictured) for $4.99
with free shipping
. (Non-members can spend $25 to qualify for free shipping; otherwise it adds $4.67.) That's the lowest total price we could find by $21. Sales tax is added where applicable. It includes a printed jumpsuit, mask, and cape.
Amazon also offers Prime members the Marvel Avengers Kids' Iron Man Costume for $5.99 with free shipping. (Non-members pay $4.46 for shipping.) That's $11 under the lowest total price we could find for a similar kids' Iron Man costume. It's available in size 4-6.