Not quite a costume contest, but last year we had some people dress up like Whedonverse characters (we had a Dr. Horrible, River, Inara, Captain Hammer...I can't remember if there were more
) and people could bid on the chance to hang out with their favorite "characters" and do little "activities" with them (tea with Inara, frozen yogurt with Dr. Horrible, etc).
It was a neat idea in theory, but did not go over well at the event. It was a real struggle to get any bids on some of the "characters," and I think that probably made people feel a little bad and took away from the fun aspect. There was A LOT going on at the event, and I think attendees were overwhelmed with options and wanted to save their money for other things.
Now that I think about it, the people we got to volunteer as "characters" were the only ones who came in costume, which is unusual compared to other years...