It depends on which "ban" you're referring to. If you're talking about the NFA, they were fairly uncommon before it because man-portable full-autos were still in their infancy and were very expensive. Other than belt-fed heavy machine guns like Maxim, Vickers, Hotchkiss, and Brownings, the only full autos widely available in the U.S. prior to 1934 were the Thompson Submachinegun and BAR which were both expensive. While $200 doesn't seem like all that much today, it was a lot of money in the 1930's (particularly when it's tacked on top of the price of an already expensive gun) and thus made legal full-auto unobtainable to all but the extremely wealthy at the time (there were very few wealthy people in the 1930's). The NFA, in effect, "killed" widespread civilian ownership of full-auto "in the crib".
Really, the Hughes Amendment in 1986 accomplished much the same thing. By closing the NFA registry, the Hughes Amendment forever limited the supply of legal full auto firearms. While firearms are durable goods, they don't last forever particularly when large volumes of ammunition are shot as is often the case with a full-auto. The Hughes Amendment limited, and over time, shrunk the supply of legal full-autos but did nothing to the demand, thus the prices have sky rocketed. To get an idea of what prices would be like without the Hughes Amendment, simply look at the prices of NFA weapons which were unaffected by it such as suppressed firearms, short-barrel-rifles, and short-barrel-shotguns. While these are still more expensive than comparable non-NFA weapons, they are still much, much cheaper than a transferrable machinegun (non-full-auto NFA weapons usually run $1,000-2,000 in my area while transferrable machineguns usually run $10,000 and up).
