I can't believe you waited until Halloween to get your costume. And here you are, desperately searching Google for good costume ideas, and you happened across this page. Well, you're in luck. Got access to a color printer? How about some sheets of sticker-paper? No? Hit the office supply store before they close and buy a 10-pack of sticker sheets that are full sized, with no crops or labels cut out of them. Sometimes referred to as 'crack and peel'. Your local copy shop might have some sheets in stock and will gladly sell you some in a pinch.
Click to Download 5kb PDF, sized 8.5" x 11", or US Letter Size. Print it on full-sheet sticker paper. Carefully cut out each emblem, then apply to your shirt, directly over your chest.
Ta-da! Instant Costume! Only cost one color print! Limited choice of super heroes, but who's choosy? Wanna be real clever, apply to your proper-colored t-shirt under your regular clothes (royal blue for Superman, grey for Batman), and give a quick strip show to thwart ne'er-do-wells with evil intentions...