Save the Children Fund and UNICEF put the total financial cost of our current suite of policies at $9.6 billion over the past four years. Credit:AP Australia's prime minister had to attempt to explain a policy that looks crazy from the outside and not much better from the inside. More asylum seekers have arrived by plane than by boat over the past 20 years, and yet it's the ones that arrive by boat who are almost always genuine. Ninety per cent of boat arrivals are found to be real refugees when their claims are processed, compared to less than half of those who arrive by air. Yet we only make life impossible for the ones who arrive by boats. Here's Turnbull trying to explain it to a disbelieving US President: "The only people that we do not take are people who come by boat. So we would rather take a not very attractive guy ... than to take a Nobel Peace Prize winner that comes by boat."

Is Peter Dutton getting value for money? Credit:Alex Ellinghausen Trump: "What is the thing with boats? Why do you discriminate against boats? No, I know, they come from certain regions. I get it." Turnbull: "No, let me explain why." Turnbull's explanation was that asylum seekers who come by boats are likely to pay people smugglers, and people smugglers let asylum seekers die at sea. Stopping boats saves lives, in ways that stopping planes do not.

The extra billions spent spent on offshore detention didn’t buy us less damaged people, they wrought greater damage. Because lives are very valuable, whatever we spend to stop the boats ought to be worthwhile, as ought whatever damage we inflict on people to do it. It's a cost-benefit calculation of the kind made all the time by governments planning new roads or railways or anything else that will cost or save lives. Yet the calculation has never been made explicit for offshore detention and the renewed onshore cruelty that accompanies it. Nor has a calculation compared it to alternative policies that might be able to achieve the same thing with fewer financial and human costs. The Opposition isn't much use. It has broadly supported what the government is doing up until this week, when it has begun to make tiny noises about the plans to end support for the Australian-based asylum seekers who've come from Nauru and Manus Island for medical treatment.

So Melbourne University economist Tony Ward has stepped into the breach. His new book, Bridging Troubled Waters, sets out the costs and the benefits of what we are doing and what we could be doing instead. Calculations from Save the Children Fund and UNICEF put the total financial cost of our current suite of policies at $9.6 billion over the past four years and up to $5.6 billion over the next four. Offshore detention accounts for 95 per cent of the cost. The Coalition's Commission of Audit found it costs $440,000 per person per year, around $1200 per day. It costs only half that, $658 per day, to detain someone in Australia, and only about $300 to keep an Australian prisoner in an Australian prison, which is about what it costs to put someone up in a luxury hotel. Processing someone's papers in the community is cheaper still, at around $250 per day. Handing out a bridging visa with support is even cheaper, at around $90 per day.

And offshore detention has other, harder to quantify, costs. You can't easily put a price on mental health, but you can work out which kind of detention damages people the most. The United Nations High Commissioner for Refugees found 88 per cent of the residents in offshore detention suffered from a depressive or anxiety disorder or post-traumatic stress. Among asylum seekers living in Australia while their claims are processed the figure is as low as 52 per cent. Ward concludes that the extra billions spent on offshore detention didn't buy us less damaged people, they wrought greater damage. But what about the benefits? As far as we know, in recent years we've had not a single death at sea. Under Labor there were 1100. But the saving of lives mightn't have been due to offshore detention, it might have been due to the (much cheaper) program of boat turn-backs that accompanied it. Few boats try to come to Australia now, even after offshore detention has been softened by the prime minister's announcement that some of those detained will be taken by America. Loading Ward reckons a much cheaper way of saving lives would be to ditch offshore detention (saving $1.7 billion over four years) move locally-detained asylum seekers into the community more quickly (saving $1 billion), to spend more on turn-backs ($11 million) and more on regional co-operation ($150 million).

It's a human and financial saving worth having. If it fails, and deaths at sea resume, we can always reconsider.