lol.....cheap compared to hunting down all the guns and taking them away from people
That's not needed. The usual approach in the US, when we ban something, is that we ban the manufacture and import of new items but we don't hunt down the old. That's how the assault weapons ban worked, for instance. It's also how it worked when we banned lawn darts, or cars that can't pass modern crash-test standards, and so on. Over time, the old weapons would become less consequential for a bunch of reasons:
(1) rising population would mean falling per-capita numbers
(2) higher demand for the grandfathered weapons would drive up the price, resulting in them congregating more in the hands of wealthier and older people, who commit fewer crimes (the same as old collector cars)
(3) they'd also tend to congregate in large collections, and even museums, where they'd be less available for mayhem
(4) many would wear out or be lost
(5) many would be seized when involved in crimes (including simply the crime of possession in the case of someone with a criminal record or otherwise not permitted to own a weapon)
It would take a while, but with every year following a ban, the guns would be less and less harmful to society. And as guns became less ubiquitous, use among the criminal underground would drop, because they'd be seen as less necessary to counter other criminals (the same way, for example, criminals in Japan are vastly less likely to have a gun, because guns aren't common in the society).
It wouldn't solve the problem overnight, but it would improve the situation a lot faster than most realize.