Banning firearms isn't "dead cheap" if you have 400 million in existence. The enforcement would cost a lot, even if you didn't perform a mandatory buyback program. Germany doesn't have an outright ban on guns, but strict regulation. Running the regulation costs money.

A note about copying - you have to look at the culture to see if how things will work. Germany has vastly different socioeconomics than the US, and socioeconomics are major factors in crime and homicide. Not to mention the percentage of people who own guns and are pro-gun are vastly different. Applying the same regulations in the US will take decades for criminals to no longer have many guns and then they will switch to knives and screwdrivers as we see in the UK, which has about dowluble the violent crime rate as the US after accounting for differences in the stats (somewhat culturally/socioeconomically similar). So it might make the gun numbers better, but it won't solve the real drivers.

The best approach would be to address the underlying factors and causes of all violence while still allowing freedoms for the 44% who do not support stricter regulation.