As I said here, a generic Worldcon budget model is maddeningly difficult to set up, on account of trying to figure out where to classify things. Should your theoretical model charge Art Show and Dealer's Room for their "share" of the exhibit hall space and decorating expenses? Practical working budgets usually don't because it's too much of a pain in the neck to administer, but if what you're trying to do is figure out how much each of your desired functions' avoidable costs is, maybe you need to do this.
Not that I'm advocating dropping Art Show, Dealers, or Exhibits, but you also have to realize that they aren't free other than once you've decided to take the large plunge of renting the space, they have relatively small additional costs associated with them like art flats, exhibit cases, and overhead hanging signs. For ConJose, those costs (which aren't really variable) amounted to less than $7/attendee.