I don't have any "inside information", of course, but my suspicion is that the total volume of .380 ammo produced is far less than the total volume of 9mm ammo. With the advent of the LCP and its competition, I am sure that the .380 production level will be growing.
When hand-loading, I find that it is a bit more difficult to produce a smaller cartridge with the same level of precision than it is to produce a larger cartridge.
- Getting the powder charges for .380 right on the money, for example, is next to impossible with many ordinary powder measures that work perfectly for larger pistol loads such as .38 Special and .45 Colt. Even with the rather expensive Redding Competition Model 10X Pistol & Small Rifle Powder Measure that I use (specifically designed to achieve the best metering possible at the targeted charge weight of approximately 10 grains), I often check the charges with my powder scale to make sure they are right on and occasionally have to tweak a charge.
- Handling the small cartridge cases and bullets takes a little bit more care to avoid problems with getting the cases into the shell holder properly and aligning the smaller bullets.
Do these things that I notice in my own production affect the automated factory production? I don't know, but I would not be surprised to see a higher actual production cost for the smaller cartridge.