The most important thing is to pick a provider that will have coverage in the area you plan to be in. The more remote the area, the tougher this will be.
I am in the US, but I would imagine that providers in AU also have coverage maps that you can check. I've been using mobile broadband for years, and I can tell you the speed and reliability varies widely - even with the same provider - depending on where you are.
I currently have a mid-level (coverage-wise) USB card for a mobile provider, and they allow up to 5GB per month. I find this to be more than enough for my basic selling activities.
And if they have the pre-paid mobile broadband for the area you will be in I recommend that highly. That way, if the coverage is not as good as expected, or if the coverage quality CHANGES (and it CAN and DOES) then you are not locked into a contract.
Such service in the us costs between $30 and $50 per month, although some charge more.
The CHEAPEST way would probably be if you have access to a phone line and use dial-up, but YUCK!!!
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