AFIK, the Thompsons used by Capone's gang were 21s, not 28s.
The 28 - with it's slower cyclic rate -came into being in an attempt to get Military sales.
Auto-Ordnance still had plenty of leftover 21s from the original 15,000 run made by Colt. Thus, the overstamp guns.
Safe to say the Thompson was not a real sales hit, even in the pre-depression days.
18k will get you a 70s production Thompson.
25 to 30 for a war two 28 or M1.
30 and UP for 21s or commercial 28s. There is a demand for the early guns and once put on the market, sale is usually the same day.