This sounds familiar to me.. As a manufacturer of something very different to shotgun cartridges, I too make a range of products, all in a similar vein, but in a range of cheaper to dearer, much as cartridge manufacturers do.
The most expensive part of the product range does have some higher quality ingredients, with more spent on inspection and quality, both of which drive costs up of course. The other factors which make the higher end product more expensive are that they are made in lower volume than the cheaper end, so certain fixed costs are not amortised (shared across the amount of product made) as widely.
Finally, the higher volume / cheaper products tend to be sold at the lowest possible price as there is more of a price-war with lower end products. So, basically it's a bit of everything accounts for the price difference in my experience..