Simple inexpensive costume that will ENSURE you get girls. I've tested it last year and it worked like a charm.
Take a large white Hefty bag.
Cut out the head and arms.
Take a large black marker and write DOUCHE BAG with a smiley face on top.
People of all kinds crack up at it. I was getting girls coming up to take pictures, people honking their horns yelling "Hey Douche Bag!", people staring, pointing and laughing. It helps to be lively, funny, and ****y. I was spanking girls at the party I was at etc. Go out in a crowded street and see how many people go berserk.
Girls will ask if you really are a douchebag to which you go right into asking for their number for them to find out.
Inexpensive, funny, and a guaranteed attention getter. Try it yourself if you don't believe me and report back after Halloween.