The cost of national eradication would be gradual, but likely, ultimately staggering. Cleaning out Rangitoto/Motutapu, even after possums and wallabies had been dealt with, cost $3.5 million, and by extrapolation the sum for the rest of the country has been put at $24.6 billion.
More recently, a project to eradicate just mice from the 22 square km of the Antipodes Islands - the million dollar mouse project - will cost $3.9 million. Extrapolating that to the 268021 square km of New Zealand would give a cost of $47.5 billion.
More hopefully, there's a 2015 paper which estimates the cost at $9.04 billion over 50 years.
One thing all of these estimates have in common is that they are in the billions. And even on the cheapest of them, the government would need to be spending a solid quarter of a billion dollars a year to meet its target. Instead, it is spending around two percent of that.
What really sucks is that it didn't need to be this way. National has $26 million to spend over five years, and (on the 2015 estimate) that's about enough to eradicate predators from Stewart Island. And if they'd announced that as their target, and clearly allocated it resources commensurate to the task, I'd have nothing but applause for them. Instead, we have a bad joke, a PR exercise aimed at getting cuddly tuatara headlines with no real followthrough. Spin and bullshit, just like everything else they do.