Cheap .50 bmg ammunition?

During WW2 the USAAF sent Clark Gable to England to make a promotional film about air gunnery. What he made was more a history of the 351 Heavy Bomber group (B17s). They did show plenty of guns and ammo and made this statement; " ammunition is cheap. 6 cents worth of .50 caliber through the cockpit of a Messerschmitt is all it takes to bring it down".

Does anyone have any idea of how many .50 BMG shells one could by for 6 cents in 1942?