I think it's because pretty much all the spin-ons can all be made on the same production line it just takes some adjustments of the tooling to crank out different size filters, the cartridges are generally newer and are made of plastic and not the same metal media cartidge designs they've used for years inside the spin-ons, so they had to invest in new tools and they had to design and test the new cartridges and I'm gonna assume that the cartridges sell at a much lower volume than the spin-ons so the costs of engineering and tooling had to be rolled into the final cost of the cartridges.10358 cartridge. It costs $2 extra to make it so small, and to omit the can, base, ADBV valve, and bypass valve. Some other cartridges are higher. Your $9 will buy a Chinese TG10358, in literally the smallest box you can find in the oil-filter section.