Mass Buy Discounts can be a part of the equation but actually somewhat of an oversimplification.
When any of you reloads do you load up 5 or 6 .9mm then run off a dozen of so .38 specials then run 8 .45acp and then 18 .44 magnum followed by 15 more 9mm?
Or do you run all of your 9mm then switch to another caliber and run that and then switch again?
Do you really think that Remington, Winchester, Federal or any other large ammo company actually has dedicated machine for every calliber they produce?
While there may be a dedicated machine for 9mm, one for .38 and another for .45 and even perhaps one for .357. all of the other ammo shares machinery. And even the 9mm machine will have to be adjusted for differing loads. You didn't think there was a separate machine for each load did you? Do you really think that they just walk into a big room dump a few barrels of components in a few hoppers an hit a switch do ya?
Ammo is produced in batches. Hopefully the company produces enough ammo to meet the demand until the mext batch is produced.
This is why some calibers cost more. it takes X-amount of time to set up the machine. if you do a set up and then run 500,000 of .45acp rounds it becomes much more cost effective that doing a setup and running 10,000 rounds of .38 S&W.