as others have mentioned, if this is the case, i'd put up the grand. that being said, you're a younger guy, and most clients understand that and don't expect you fork out a lot of money. if you can, great, but if i were in their shoes, i'd be happy that you came and contributed, no matter how much you gave.
edit: it seems like a formal event, so i'd probably go with a suit, and that's coming from a guy who almost never, ever wears suits. you can even go mid-way and wear a matching pants/jacket combo with no tie. ( a less formal suit...i tend to get them at banana republic because they fit well off the rack).