Need A Good Wedding (Attendee) Dress?

In three months, my stepdad's sister is going to be married. I asked her about what I should wear, as just a normal guest, and she said anything that's fancy. I consider myself gothic/steampunk, and was wondering A) should I tone down to a nice ladylike pastel dress? and B) Where can I get a good dress thats gothic, fancy, and not three hundred dollars?!