The Australian opposition has dismissed the government estimate of the cost of restarting asylum-seeker processing on Nauru.
Its leader, Tony Abbott, has described the figure of two billion US dollars as completely fanciful.
And its immigration spokesman Scott Morrison says the government has gone for a hyped-up, electrified Alcatraz alternative.
The figure was given for dealing with 750 applicants over four years.
There is an intensified debate among politicians in Canberra, who are considering reviving a 2001 policy of processing hundreds of Australia's asylum seekers abroad.
The Australian approach has been widely criticised, including by the United High Commissioner for Refugees.