Although I do make some phone calls and texts, the amounts are negligible. I tend to use only mobile data, so when I shop around I look for the cheapest possible pre-paid mobile data. Currently in Australia, this seems to be Amaysim. For $100, they provide you with 10GB of data for one year. However, there is a catch.
Rather than billing customers according to kilobyte, Amaysim bills customers according to megabyte. For example, if you send an email or Facebook message, you will use about 4KB of data. Companies that charge you according to kilobyte will bill you for 4KB of data. However, Amaysim rounds this up to 1MB (about 1000KB) and charges you for that. This practice has been exposed in this Sydney Morning Herald article: Telcos take a byte out of pre-paid mobile deals.
SMH summarises the charging practices of telcos below:
Vodafone - Will charge in 1 kilobyte increments from February 13 on all pre-paid plans. Was proposing 1 megabyte increments across all plans before it about-faced.
Telstra - Charges in 1 kilobyte increments on most pre-paid plans but does charge in 1 megabyte increments on its "Simplicity" plan.
Optus - Charges in 1 kilobyte increments on 1 pre-paid plan and 1 megabyte increments on 2.
Virgin Mobile - Charges in 60 kilobyte increments on all pre-paid plans.
Red Bull Mobile - Charges in 250 kilobyte increments on all pre-paid plans.
Woolworths Mobile - Charges in 1 megabyte increments on all pre-paid plans.
Boost - Charges in 1 megabyte increments on all pre-paid plans.
Crazy Johns - Charges in 10 kilobyte increments on all pre-paid plans.
Amaysim - Charges in 1 megabyte increments on all pre-paid plans.
Kogan Mobile - Charges in 1 megabyte increments on all pre-paid plans.
All this makes it difficult to compare among different providers because you are not comparing the same products and therefore you cannot compare by price.
Even though Amaysim rounds up to the nearest megabyte, the other providers seem to charge much more than Amaysim and even for pre-paid they charge fixed amounts per month. I will stick to Amaysim for now, paying $100 for 10GB and try to limit my data usage to 800MB per month, even though they round up.