If everyone is dressing up, improvise man. It's better than going as nothing, because costumes are an automatic opener where you're guaranteed a minute or two to build up rapport. It's hard to do get in on that if you're not dressed up. Get your bathrobe or borrow someone else's, get a cigar, and go as Hugh Hefner. Hell, one year in college, I just wore my bathrobe with boxers and slippers, didn't shave, and said I was going as "Lazy" - that night rocked

Another year when I was living off campus and visiting a buddy at the dorm, I just borrowed his karate gi. The lameness of the costumes can help you come off as too cocky/cool to fuck around all day spending your money on a stupid costume.