According to whistleout.com.au, when pricing for NBN services is compared on a per GB basis to ADSL2+, it's going to be cheaper once line rental costs are taken into account.
Once you look into the whistleOut research, that does pre-suppose that you're comparing something like apples and apples. If you choose the fastest speeds or 100Mbps from the NBN, you'll pay more than many of the current ADSL2+ plans offered by companies that have announced NBN plans.
One of the interesting bits of data that's in the whistleOut report is that it would take about nine days to download 1TB of data using ADSL2+. That drops to just over seven hours with the fastest NBN speeds. Given that we live in the age of terabyte storage, there's a compelling case for the NBN there for our online backups.
There's loads of interesting information in their study. You can read all about it at WhistleOut.
NBN Pricing Analysis vs ADSL2+ [WhistleOut]