Hi Leon,
Whirlpool folks have created a spreadsheet comparing all carriers:
http://wizard.id.au/spreadsheet/aust...an-comparison/. This will give you a good overview of the cost.
Please note, if you are after full Telstra coverage, then only Boost Mobile offers full access to the Telstra network. The rest of the Telstra resellers offer Telstra Wholesale network access, which is a slightly limited in coverage in rural areas and also in speed.
If you want to test how different networks fare for your location(s) and usage, you can always grab few prepaid monthly sims from different providers and try them.