well RAW would be to take the base price of an alchemical paper cartridge (12G presumably) and apply the special materials rules (see core rulebook @ page 154 or 1/2 way down the equipment section in the PRD) to find the cost of the cartridge when purchased from a vendor. Use the gunsmithing rules (provided you have the required one level of alchemy) to construct alchemical cartridges for half the cost of "store-bought" ammo.
Using RAW:
adamant alchemical cartridge costs ( 12G + 60G ) / 2 = 36G to make
alchemical silver alchemical cartridge costs ( 12G + 2G ) / 2 = 7G to make
cold iron alchemical cartridge costs 12 G x 2 / 2 = 12G to make
This makes alchemical silver alchemical cartridges much cheaper than using the house-rule plain silver alchemical cartridges. A store-bought silver alchemical cartridge using the special material rules costs 14G which less than the 25G for a silver bullet alone on the table from the firearms rules. The 25G silver bullet price was probably adopted to handle the gunsmithing 10% RM cost to manufacture bullets & powder and put silver bullets made by a gunsmith in line with silver arrows and silver sling bullets made using the 1/3rd RM cost regular craft rules, something the devs seemed to have overlooked with the 61G adamant bullet (only 6.1G to make versus a 20.1G adamant arrow, and even an adamant alchemical cartridge only costs 12G to make which is way too low).
How I would house-rule is to use the regular special material rules for ammo ( and reducing the silver bullet to 3G on the chart) and modify the gunsmithing rules to craft bullets made from special materials at 1/2 cost for raw materials instead of 1/10th cost for raw materials. Otherwise there is little price disincentive keeping a gunslinger from always using adamant bullets.
