It's pretty much the standard model for all dial-up connections - pay so much per month, get so much free. Or you pay a higher monthly fee for "unlimited" service.
With Cable there's basically Optus and Telstra. Telstra's most popular Cable plan is 3GB a month, and 11c per MB thereafter (I think). Optus (currently) have a much better idea - use what you like, but don't overdo it. If you're using more than ten times the bandwidth of everyone else, you're kicked from the service. It's on a two week rolling average, and you can check the average at any time. Funny in a way - kinda encourages everyone to download as much as possible so the average is kept high :)
ADSL - the general consensus still seems to be It's Too Sucky. Expensive, and low quality service.
My ISP (dial-up) changes $9.95 a month, for 80MB a month. It was 100MB but they just dropped it. After that it's around 17c a MB I think, but a monthly bill will never rise above $29.95 a month.