According to wikipedia it cost Nextgen $850m to build their network over a 15 year timeframe. Vocus paid $861m, it was originally reported it would cost $807m.
That figure is a bit fuzzy, its value depreciates over time, but substantial operational parts would be expected to have a 25 year life. It also doesn't consider the risk value, if you buy an asset already completed you aren't taking a construction risks, so you should expect to pay a bit more.
It also doesn't consider the strategic value of the asset, there is only need for a certain amount of national fibre networks, there has to be enough to satisfy government that there is competition, and not so many that they become unprofitable. It would have been costly for all players in the long term if Vocus to built a new network to compete with all others.
It also doesn't consider the timing of the purchase in the growth of Vocus, it was an important piece of the puzzle at the right time, just after expanding our retail base with Dodo and Iprimus, and Nextgen is still expanding to Indonesia and Singapore, which will be the first overseas link we have built. It gives as credibility as wholesalers which is going to become increasingly important as NBN only resellers start competing for market share.
Vocus raised $648m to pay for Nextgen at an average share price of $7.79, thats $285m more than they would have been able to raise at today's share price. Which is cold comfort to people who paid purchased shares at those prices, but it means Vocus today is a stronger company, it has $285m less debt because of the way the purchase was financed.
Some may consider that Vocus paid too much, $11m more than the cost it took to develop it, a 1.2% premium, but that is a very naive way to look at it.