Depending on the game setting.
It should always be an Advanced Skill, even if Craft isn't (or is it?).
Gloranthawise, ironmaking has become steadily less available - in RQ2, most cults had access to iron weapons, in RQ3 only a few cults got Enchant Iron, in HQ Iron is incredibly rare. Who knows how rare iron is going to be in this version of RQ.
Traditionally, the Mostali have had iron, the Third Eye Blue cult stole it from them and became itinerant ironworkers and certain main cults taught Enchant Iron and Craft (Ironsmithing) or Craft (Blacksmithing). So, cults such as Gustbran, Humakt, Orlanth and Yelmalio have Ironsmiths to make their weapons.
So, one way of doing this in RQM is to have Craft (Weaponsmith) as the base Craft which teaches how to smelt and work bronze and then have Legendary Abilities of Ironsmith, Goldsmith, Silversmith, Leadsmith, Coppersmith, Aluminiumsmith and so on. You could even have Dragonbonesmith, Bonesmith, Woodsmith and so on to cope with other materials.
As to their costs, they shouldn't be that expensive but they are specialised, so they shouldn't be cheap either. Perhaps 6 Hero Points to learn with the requirement that the person be a Smith or metal worker of some kind.
They would be available through cults, guilds and secret societies.
Outside of Glorantha, you could still have the Legendary Abilities, but most Blacksmiths would start being able to use Iron and then would have to buy the other Legendary Abilities to be able to work with other metals.
[Material]smith
Hero Points Cost: 6
Requirements: The character must be a smith or metalworker
This is available in several forms, each of which must be learnt separately. So, there are Ironsmith, Leadsmith, Aluminiumsmith, Coppersmith, Goldsmith, Tinsmith, Silversmith and so on. This gives the ability to work with the stated metal or material and to use other Craft skills to make weapons or items from that material. So, a smith with Ironsmith will be able to use his Craft (Weaponmaking) to make iron weapons and a jeweller with Goldsmith can make gold jewellry with his Goldsmith ability.