One thing to look at is the number of people involved in running a WorldCon - for example, https://chicon.org/home/about-worldcon/chicon-8-committee/ - there are easily over 100 people involved. I can't think of too many places in the US where we have a club, or clubs, with enough members to get 100 volunteers.
And they all purchase memberships to the Con to become a volunteer. Yes, they may get all of that refunded, but it may not happen.
As for the dealers, they pay $350 for a single 72" by 30" table with two chairs and a membership for the person at the table at $690.00 for DisCon III - there are some discounts for additional tables & memberships, I think.
Also a number of past WorldCons have been part of established Sci-Fi cons which have run for decades.... I'd also look at the list on http://www.smofinfo.com/LL/TheLongList.html and look at the number that attended and the number that were members for each year.
While there are some ideas we might be able to work into the Nats, WorldCon is really a different animal....