Another thing about the NBN costs is that many of the costs must be expended anyway.
For instance, the current broadband over satellite for the bush service is currently getting close to End Of Line. It will require about $7b from memory to replace it. These costs have been moved into the NBN, but will have to happen even if the NBN does not go ahead.
Another of those costs is the cost of Fibre to regional centres. Once again, this has been moved to the NBN, and is needed anyway. Lets put a price of $1b on it. So we have about $8b of government costs, even if the NBN does not go ahead.
Then Telstra would need to spend at least $10b upgrading it's copper network over the next 10 years, because it if falling apart. This puts the total to about $18b that needs to be spent regardless.
On the other side, from the FIOS experience in the USA, I predict that there will be about four house fires during install caused by installers hitting gas or electric lines. I hope I am wrong, but this is a prediction.