While not double, (current exchange rates show that 99.95 $AU is equal to 84 $US today) I can only tell you that pricing is set by the Regional offices based on any number of factors. I don't know what any of them are, but speculate that it could be support costs, competition, infrastructure (local) and even taxes. The price via download is the retail list price, but you may be able to find it discounted from a retailer. Amazon, for example, sells OneCare for 1/2 the US list price.
-steve