In 1928, the standard M1921 TSMG sold for $175, $25 extra for the Cutts Compensator. One 20 round stick magazine was included. The M1928 sold for $200, $225 with the Cutts and one XX stick.
(FWIW, the TSMG magazine letters are the capacity in Roman numbers - XX, XXX, L, and C.)
Millions of machineguns prior to 1934? Not hardly. By 1938, only 10,300 Thompson SMGs had been sold, and only 4700 in the US. That counted guns "in the wrong hands." By December 1934, the end of the registration period, only 15,791 machineguns and sawed off rifles/shotguns had been registered. (At that time, law enforcement agencies did not have to register their NFA guns.)
There was almost no collector interest in machineguns; most WWI "bringbacks" were in the hands of veterans organizations as war trophies, not as collectibles.