That's certainly how it works for most Australian ISPs. And the off-peak times only run from about 02:00 to 07:00. I'm lucky enough to have an unlimited ADSL account with TPG, or my quotes would be a couple hundred GBs for the same cost.
Luckily, I think competition is going to be making things better for the Australian ISP consumer. That, and recent improvements/additions in undersea cables.
I can't speak for your experience with TPG, but a lot of users I know on TPG hate the fact that they have "unlimited downloads" but in reality get limited bandwidth due to an oversold service.
I decided I prefer a limited plan where I know what service level (particularly latency and throughput) I can expect. For instance, plans with Adam and Internode are not that much more than a TPG plan for more usage than I'll forseeably use (I get 500gb a month or so at 20mbps + VOIP for $100), but the difference in gaming latency and actual speeds are both considerably more stable and faster than friends on TPG.
My experience with TPG has generally been very good. I connect at 17-18Mb/1Mb and consistently download at 1.0+ MB/sec. Usually 1.4MB/sec, though I've seen it drop to ~800KB/sec.
Given that your plan is 60% more than mine, I'll gladly pocket the difference and live with the minor problems I've had. As another poster mentioned, I chose to vote with my wallet. Generally small quotas + absurd off-peak times (where the majority of the quota is allocated) make my blood boil and I'll support anyone who helps break up the status quo.
Off-peak times vary betweem ISPs. But you can also vote with your wallet. I'm with Internode, which is a little more expensive, but they don't have peak/off-peak (among other advantages, for me at least)
It doesn't translate to direct costs, but my speed is largely dependent on contention with other users, and drops heavily during 'peak hours'.
Then again, I suppose you could argue cost per GB = integral(download speed * time) / cost per month, so 1GB at slow speeds is costing me more in terms of time-bandwidth product at peak rates than otherwise. Soft data caps and peak-time QoS might also be happening behind the scenes, skewing matters more.
Cost per GB isn't really even what you want to measure, and doing so is what causes so much confusion.
Let's say you're an ISP and you have a 10Gbps connection to the outside world, and the routers etc. that can keep up with that speed.
To a first approximation, your costs to serve up 10Gbps to all the customers in your area is 0 (you've got to pay for electricity and engineers and etc, but this doesn't vary with user activity).
If your customers in your area all queued up nicely and downloaded stuff at full rate, never overlapping with each other, you could transfer 10Gbps * 1 month for basically nothing.
However, your customers all want to do stuff at the same time, and thus contend with each other for a slice of that 10Gbps. If you want to serve all of their requests, you're going to have to spring to upgrade your total capacity.
If you don't own your own fiber, you have to rent a link from someone else, or if you do and there's no more available you have to physically come up with more (and pay to dig up the road and etc.). Now your routers and etc. might not be able to keep up with your new backhaul speed, and you have to upgrade those...
You really want to measure "Marginal cost of +x Kbps during the busiest typical hour."
Probably quite a while. Phone companies have done and continue to do this, but are moving more and more away from it.
I'd be thrilled if someone tried it, though. It's more honest and more likely to address bad user experiences than arbitrary "bytes transferred per month caps."
That's how http://aaisp.net is billing (but they're quite nerdy in general, e.g. you get your own IPv4 with personal RIPE entry and RevDNS + IPv6 ranges)
