http://www.kotaku.com.au/2009/05/monday-musings-the-price-is-right/ is a nice article about it.
Essentially, the reasons include:
1) Taxes on imports put in by the local government
2) Market conditions - they can get away with it
3) Increased cost of supply due to economies of scale
4) Global fluctuations in monies. Ubisoft deals in Euros while Bungie deals in dollars. Usually, the Australian branch reports to the home office and accounts in that currency. Since the Australian dollar can fluctuate, the prices settled at $100 AUS to accommodate both feast and famine.
A similar situation exists, for example, in Mexico. New video games are sold in Mexico for approximately $80-100 USD after the exchange. First, there are taxes. Second, there's an economy of scale in the US: greater demand, greater sales numbers, etc, able to allow a lower cost. And third, video games in Mexico are pretty much the purview of the rich and of Americans there for a brief period (such as me), so the companies can afford to put off people who can't afford their products anyway and charge those who can more. It's called price discrimination, and it's something companies do all the time. It's not always to your disadvantage; military and student discounts, senior citizen discounts, early bird specials, and special group rates are the same thing. You charge different prices based on not only on costs of production, but on all parts of supply and demand, and part of demand is motivation of groups of clients.
For the record, I'm -not- complaining about the higher price I'm paying in Mexico during my stay. It's my choice, pay it or not. I just change my spending habits. Instead, I'm buying more tequila and less video games. I don't need video games. I -like- them.
And today's prices aren't that bad; NES games used to cost $50 USD back when a new luxury car cost $8,000 USD and gas was under a dollar a gallon. Prices have gone down, adjusting for inflation. I remember paying $80 USD in the mid-nineties for Final Fantasy 3 (yes, FF3) on my SNES.