Banning manufacture and import is functionally a ban. You are simply disputing semantics. If you want to repeat the steps that have led to a fully auto costing $10000 and being out of the hands of criminals for other firearms but call it "Oktoberfest" instead of "Ban" I'm entirely okay with it, the semantics don't bother me. In fact, it will probably make it an easier sale to the public.
Wait I thought we agreed that was a legal fully-auto? Someone go check silk road for the black market price...
The legal price is relevant because guns in the US that end up used in crime are often legally purchased originally. We don't have many, "Oops, my fully automatic AK-47 got stolen, Officer!" when it costs 10k and the number of available guns is finite. A rich collector will pay more than a bank robber because the bank robber will just use a cheap handgun or (rarely) semi-auto rifle so the fully-auto guns stay off the black market.
I would be interested in the black market price in the US if someone can dig it up, but again there just isn't that much demand. The country is flooded with tons of guns already and large scale importation of illegal firearms is (relatively) difficult because of serious port security. There is a much bigger market for these weapons in the developing world where people actually fight battles with them. In general, the US illegally exports firearms far more than it imports them.