Cold Iron weapons?

[Giga]

What do they do and why are they so expensive?

[Kommissar K]

Cold iron affects a certain type of DR, that some creatures might have. DR stands for Damage Reduction, and is a flat decrease in damage they suffer. So a creature might have DR 15/Cold Iron, which means they reduce the damage they suffer per hit by 15, unless its dealt via Cold Iron.

As for why its expensive... https://www.d20pfsrd.com/equipment/special-materials/#TOC-Iron-Cold

Really its one of those "just because" things. Likely its so much because there isn't really a downside, as opposed to say, a silvered weapon.

And not to be too elitist, but wait till you see the pricetag on +3 weapons.

A 2gp dagger costing 4gp to be cold iron is a pittance. And its absolutely worth having as backup just in case.

[Randomz]

In a world sense its because:

"This iron, mined deep underground and known for its effectiveness against demons and fey creatures, is forged at a lower temperature to preserve its delicate properties."

It's a rare metal that's hard to work with essentially.

[Myst]

Buy one before you go to Bridge Over the Gudrin River, or you won't have a fun time.

[Kosiciel]

Let's not talk about adamantine weapons, then...

[wendigo211]

You can get a +1 cold iron dagger in the dungeon under Old Sycamore. I think it's the Hermit Mite in the lowest level?