Conventions are really expensive to run. In general they are a business. Things like outside food may be regulated by the hotel. Most hotels want you to buy their high priced crap. For instance, I was at Templecon this past weekend and the hotel was selling soda at 4 bucks per 12oz bottle. In the contract with Templecon it was stated that no other vendors could sell soda.
Cost wise the ballroom area at the hotel Templecon was at which was large enough for 500ish wargamers was 10k to rent for the weekend.
My suggestion would be to find someplace cheap and start small for a year or two. Get a player base and then expand. When I first ran Gaming for Giving (which we ran for 5 years) we found a Mason's hall. We were able to get the space for 500 bucks for the weekend. The first year we had 75 people and ran Warmahordes, D&D (with an official GM) and Board Games. By year 5 we had 300 people. The growth was mostly word of mouth of people saying they had a good time.
I know the people who run Templecon. They ran it at a loss with money out of pocket for the first 3 years. Last year they had 4200 people in attendance (this year was 1500 but that's a totally different issue and probably a rant
). So be prepared to put in 100's of hours of work and get nothing back for a while.
Have you looked at NoVa (it's this coming weekend)? I went last year and it was a blast. The cost was 85 bucks after I entered all the events I wanted to get in to. There was tons going on and all the people I played against were awesome. The prize support was amazing.
If you have any specific questions let me know. If you decide to go forward with it and need contacts at different companies and vendors let me know, I know people inside of the business at lots of different vendors from hanging out at conventions over the past couple decades
