Depends on the wedding. Sometimes it's the dress... but it's usually a toss up between the dress and the food.
My dress was $750. My dinner for 80 people (app, main, desert, wine, coffee) was $2700. And I wasn't even charged full price! (extended family! :D)
A friend of mine got married in the fall.. her dress was $1400, and her dinner (app, soup, turkey - omg it was good - dessert + wine and coffee) for 85 people was $4300!