In terms of time - school holidays
In terms of cost - it depends on guests, but I've never really been convinced to go to Supanova, so take that as a guide as to how much I'd spend. Probably less if it involves travel to Melbourne.
In terms of guests, I'm not really fussed, as my itch is scratched by getting the read the reports of international guests on the international sites. Maybe people specific to the Australian section of the franchise?
Supanova's more expensive because it's a really big convention with a lot of international guests. If we're talking about a smaller scale convention with fewer guests, then I'd say the pricing of the Heroes and Villains convention is more reasonable (e.g. $25 for a 1 day adult pass).