I've been involved with running a large, multiple day event for the model horse hobby for several years. The big issues are that you need to be in a place that a lot of people can/are willing to get to easily. There has to be something for non interested SOs and parents to do. You need to attract a lot more than just the hardcore people, because there are not enough of them to cover costs of the event.
The show I have been involved in regularly loses between $2000-$10,000 depending upon the location and time of the event. I have not seen numbers for the 2011 event, but I am pretty sure we went over $10K in the hole on it.
We generally hold an auction or raffle, but those can have multiple problems as well. Running one of these legally with less than a year to do so is hard-every location has it's own set of rules and costs. The time we ran in Las Vegas, our entire auction had to be of the silent variety because you need to use a licensed auctioneer in Vegas, which cost $1000 at the time. We tried to figure out a way to put a volunteer in this post, but they had to have schooling and between that and the license fees (oh, yeah, and the time to do it) it quickly got cost prohibitive.we'll never know for sure, but a live auction generates excitement and bids in ways a silent one just doesn't, so we think that auction didn't bring in what it could have. Our last auction, btw, has resulted in some super fun attention from lawyers when the person running our auction refused to remit expenses properly or ship items as she was supposed to, and that cost money too. And still is, as more than a year after the auction (July 2010) it's still unresolved.
While I love the idea if a con, putting one on for a small group is going to be a huge money loser. It might be a lot more feasable to just hold a get together at someone's home or local VFW hall
"Positioning is everything in life." A. de La Hunta