Last year I couldn't afford a costume, so I put a box on my head and drew a smiley face. Everyone liked it.
This year I may do the same thing.
You might as well attempt to dress as the TF2 Soldier's robot costume while your at it.
You just need: Tape, Two red solo cups, a hanger, two dryer hoses, two shoe boxes, a box for your head, a maker and scissors, and you're all set!