Cost of the plant was around $4 billion and the 30 year operating costs are apparently around $1.5 billion.
It was only finished in 2012 and so far it has not been required to be used.
Over the course of the drought the government handed out $4.5 billion in drought assistance to farmers and the economy overall had over $5 billion in losses per year. 2002-2003 alone cost $6.6 billion.
To be honest, this looks like a good insurance plan. Australia should build more of them.
Over-building on infrastructure is almost as bad as not building it at all. And as someone else pointed out this particular project was beset by dodgy builders, unions drinking at the trough and grandstanding politicians who wanted to be seen doing "something". All up, massive waste of money.
Edit - I am not sure I agree with you about overbuilding on infrastructure though. A good example of this is the London sewer system by Joseph Bazalgette, that was massively overbuilt to the tune of twice the maximum requirement from what was considered the highest possible population density, just in case. Which was very lucky, given the development of the towerblock 100 years later.
http://www.theage.com.au/environment/water-issues/desalinati...
We've now got two desal plants here that currently provide 27% of our supply. The supply drawn from dams now only makes up 33% of the total.
The plan is to transition to a 50/50 mix of desalination and underground sources (with groundwater replenishment) by 2030.