Josh, I'm in Sydney, so I understand the pricing issues. But, when you say it makes no economic sense, then you're not looking at the full economic picture potentially.
If you're selling your work, then ink should be one of the smaller components of the cost. Unless you're actually going backwards, you need to decide on the value of having local support, local distribution and so forth. How long does it take to get ink from the US compared to a local dealer? I know a lot of the US dealers ship very promptly, but they'll never beat going into the local and buying immediately. If you have a fault, you've got no local warranty and if the local dealer decides to shut up shop you've got even less options. Are all of the dealers able to guarantee to ship to Australia? For shipments over $1000- you're liable for GST, so add 10% (avoiding it just risks the ATO stinging you with a fine if you get caught). Etc etc - I'm sure you've done all those numebrs.
The cost of doing business in Australia is higher than the US and the currently flagging USD and strident AUD is making it even more apparent.
Of course you don't want to pay any more for something than you have to, and competition is good, but if you're a professional then you're buying more than just the consumables. It may still be worth it, but again that 40% saving in ink is not a 40% reduction in your total costs - it's likely less than 5%, which is not really a big margin to pay for local support and distribution (which again is not to say some pressure on local prices is a bad thing - I love competition and cheaper prices as much as anyone :-)
Ultimately you're the only one who can decide what's right for you, so I'll never say you're wrong, but just consider the wider picture.