Sure, but it wouldn't last. Shrink that much and the price would drop really significantly, down to below $100 -- think BayCon + 20%. Otherwise, the Worldcon would start making absurd amounts of money, and fandom won't stand for that. If the price dropped that low, the demand currently being choked off by the price would shoot up and we wouldn't fit anymore.

The only way to have a small American Worldcon and a low price would be to put a membership cap on it. Do that and people would just start buying memberships cheap and reselling them dear due to the excess of demand over supply.