Ooh, that's an excellent point! State fairs never cost US$600 per person, even when you include all the funnel cakes and donut-bun hamburger sandwiches bought from the concessionnaires. Typical admission prices in the US are maybe US$20 per day? But they have plenty of space, lots of conference rooms, bathroom facilities, handicapped accessibility, folding chairs, and so on. I bet you could hold an academic conference at a state fair fairgrounds for under US$100 a person. But I've never heard of a sci-fi con doing this, so maybe there's a fatal flaw in this idea I'm not seeing.