It's a common argument that illegal immigrants, as a whole, are a drain on their host nations' economies. Do you think it's possible to successfully divide them into categories, given that their very nature is that they're undocumented and it's not easy to pull out any records on them?
No, and I can see how what I said might have been somewhat unclear. I'm not suggesting that we don't prosecute people who are found to be illegal immigrants based on information procured with a warrant. I'm suggesting we don't go out of our way to look for them without due cause for suspicion; in effect, treating all illegal aliens the same way, regardless of whether they do anything to attract negative attention.
The reason why I included the cost of deportation in my list of costs to weigh any benefits against is that, significant or not, it is part of the net cost. If it were the only cost, then it would become easier to justify deportation, as the situation would be following issue of a warrant. But it's not, and the total cost of the operation needs to be weighed against any benefit to society.
Until that justification can be made, my default position is to support government inaction -- the government should not take poorly justified action just because there is no better option. There is no mandate that the government be involved in every aspect of society. The widespread belief otherwise is one of the biggest problems with modern Australia.
As I said, I think the best permanent solution is to make it easier to enter the country legally. By removing the incentive for people to sneak in, immigrants become better documented and thus better understood, and their income becomes taxable. We certainly shouldn't be making up for our supremely overcomplicated immigration process with supremely overpowered policing.