Since this is a U.S. militaria forum, I suspect that a U.S. military Class 3 weapon would be desired. I believe that the Reising s.m.g. (M-50) is probably the cheapest submachine gun/machine gun available, with prices in the $3-5,000 range. Parts are getting scarce for these weapons and original 20-round magazines are selling (when you can find them) at "collector's" prices (in other words: REAL salty!)
The so-called "license" is really a transfer tax.
I wonder how many Thompsons, etc. are out there unregistered due to the Amnesty not being well publicized? I suspect that a number of DEWATS went unregistered. . . Hate to date myself and almost hate to remember the inexpensive DEWATS that were available: MP-44/45/StG-45 for $49.95; Thompson M1A-1s for $75; Thompson '28A1s for $95; Johnson M1941 lmgs for $59.95; M-3 "Grease Guns" for $49.95. . . Excuse me while I wipe the tears from my keyboard!
Chauchats were $9.95, now selling for $5,000 (if I recall). And, yes!, the Chauchat should be considered a U.S. military arm (World War I). BTW, although Common Wisdom really criticizes these weapons, some of that criticism may be unjustified. Oh! Don't forget the Hotchkiss, also a U.S. weapon!
I don't want to think about the current price for Lewis guns! If I recall, they were available as DEWATS for around $50 to 75. And drum magazines for this weapon were about $9.95!
If only Santa could bring us low priced, classic full autos -- even DEWATS!
Forgotten Weapons did a video where they reviewed and fired the Chauchat, and it drastically made me reconsider my previously negative opinion. Not the best by a long shot, but for its time, not too shabby as long as the mags were clean and it wasn't one of the US versions.
I think the only real major criticism I could still apply is the aluminum barrel jacket inside the body tube of the weapon. Around 300 rounds, they said, and it heated and expanded enough to jam the gun out of battery. Sustained fire wasn't its purpose, but still, that just seems like a poor idea!