Some caliber are now relatively cheap such as 9mm and .45 acp but when you get into .45 Colt, .380 and certain rifle calibers like .38-55 and .45-70 they become quite expensive.

It's also not easy to find certain calibers on the shelves of stores. Rarely do stores (especially the larger ones) carry .45 Colt or 38-55 and if they do they usually only have one offering at that!