The value of a wall of iron

[Jack Simth]

Remember also that this is a sustainable and environmentally friendly way of getting iron.

Well, less un-friendly. After all, you still need to get and refine the gold for the Material Component. Usually much less impact than the amount of Iron the spell creates, but it's not perfectly sustainable.

 

[moritheil]

If you really want to think of ecology, eventually all that iron continually appearing in one place will form a fertile place for some sort of super rust molds or such to grow. Eventually the area will be completely overtaken by the stubborn growth, and will have to be abandoned (all metal there is contaminated, including the PCs other gear.) The mold gradually eats away at metal, even enchanted metal objects.

Soon it spreads unchecked, traveling by the great economic propelling power of cheap iron being bought by local merchants who carried it to sell elsewhere. Oh no, now the entire world is threatened! Metal items everywhere are corroding to ruin, and it's all as a result of the PCs actions! The world collapses, in chaos and fire.

And the DM dusts off the Athas setting and looks in it for inspiration . . .