I suspect we in the 1970s and on got used to prices on government contract overrun production sold on the civilian market. Get a situation where the government is taking all it can get including civilian production, you get ammo shortage. I have lived through a few over the decades, but none like the current shortages in civilian shops.
9mm ball would not stay cheap forever. In a stable market, no periods of government contract overruns, no periods of government ordering all it can get, just production passed on demand and price of materials, 9mm price would stabilize above .380 and below .40 S&W on material, adjusted for economy of scale (volume of sale and steady demand can reduce price: see .22LR in recent years selling for less than .22S or .22L today, wheres as in generations past when there were equivalent volume of demand for .22S, .22L and .22LR, .22S was cheaper than .22LR).