Ahh Telstra. Love 'em or hate 'em they do have the best network. Unfortunately.
To be honest sometimes with tablet data you can get away with using something like Amaysim (who operate on the Optus network) and it's not too bad at all. Certainly cheaper. But if you're dead set on Telstra for the better reception here's some simple maths for you.
|Cost $||Gigabytes||Days to use your data||Cost per gigabyte||Notes|
|$15.00||1||10||$15.00||10 days to use? WTF?!?!|
|$30.00||2||30||$15.00||Getting better. I guess.|
|$50.00||4||60||$12.50||OK no we're cooking.|
|$200.00||12||$365.00||$16.67||The most expensive because you get a year to use it? Come on Telstra. Get real.|