Con-News has a valuable article by Petréa Mitchell that analyzes why using a convention center is an expensive choice for an sf con. She explains the less obvious facts of life in an entertaining and knowledgeable way. For example:  

WiFi. Your attendees expect it, and in a geek-intensive community like fandom, they know that it isn’t that difficult to provide- in a technical sense, anyway. Some actual numbers: Last year’s Worldcon was looking at $36,000-$45,000 for the necessary access points, which is why it didn’t have free WiFi. The one in 2006 had a wired connection (if memory serves) to its Internet lounge, and that cost $38,000.