To craft a low grade cold iron weapon, you must be an expert in crafting and have the appropriate materials and formulas. Here is where the confusion is happening.
Do I need
a) a formula for low grade cold iron weapon? So if I want to upgrade the grade I would need a new formula and for a different material I would need a new formula.
b) a new formula for a low grade cold iron (EACH INDIVIDUAL WEAPON)? which means that I would need two formulas to craft a low grade cold iron DAGGER and a low grade cold iron RAPIER?
I would say if a PC has a crafter’s book and knows how to work with one grade of one material, they should be able to craft with an item of that material.