I want in. I'd love to have a Cosplay Ball... Even if this thread is quite old.
If you want prizes and stuff you can always call places like JB Hi Fi, WOW Sight and Sound, Borders, Dymocks and anime shops round Brizzy (etc, etc) to see if they'll donate. I dunno if you'd be able to get vendors at first but if you do it more often then you probs could.
Also, if it's a sit down meal thing, you have to make sure everything meets health & safety regulations which means it might be easer if you got a hall/function room with the option of catering?
I've never been to a ball before but to me it sounds very formal and some cosplays well... aren't formal. Would people feel more comfortable if there was something else to do besides dancing/eating/sitting? A mini cosplay contest could be an idea, maybe some games/karaoke but that sought of makes it more of a get together than a ball... All depends on how you want to do it.
I don't really have experience organising things but i just thought some of these things would need to be considered!! XD
The power of Cosplay compels you.