In Canada they spent about $2 Billion dollars for their firearms registry. For that $2 Billion dollars they registered about seven million guns. That is about $285 per gun. And they probably didn’t get more than about half of them. There was widespread disobedience.
In the U.S. there are about 300 million guns in circulation. Assuming the cost of registration per gun does not go up (and I have some novel ideas for increasing the cost that we came up with when the issue came in up Washington State back in ’97.) The total bill for firearm registration would be over $85 Billion.
Expect that price tab to be very, very conservative. Not only was the Canadian effort implemented nearly 15 years ago U.S. gun owners might not play nearly as nice as the Canadian gun owners did.