Originally posted by Mr B.:

Once you get the gear, Iron is far more of a ball ache to get. If your base is near the plains, you accumulate green metal by the bucketload.

I consider iron more scarce than green metal.

For me this is mostly because there isn't a ton of recipes that use black metal. It feels like iron is used in everything. As soon as I got black metal chests I replaced all my reinforced chests with those instead which recouped some iron.