Wall of metal let's you pick different materials for the wall, but it basically tells you to go find out what the statistics of the wall is. I did some digging to figure this out.

Logically, wall of metal, a rank 6 spell, should be a bit stronger than the rank 5 wall of stone. Wall of stone starts at Hardness 14 and 50 HP. I searched around a bit and found the material statistics, and according to that something made of stone with 14 Hardness and ~50 HP is a "stone structure."

Now, wall of stone is 1 inch thick, and wall of metal is 1/4-inch thick. I don't know if that's the threshold for whether something is considered a "structure" or not, but for the sake of argument I'm going to consider the thickness of the wall to be negligible in this instance. Literally JUST saw that the spell says "use the statistics of a structure of the material you chose." So my assumption was correct. Nevertheless, I'm still going to continue so all the numbers are in one place.

Anyway, using the "[Material] structure" as a baseline, I figured out what the intended Hardness, HP, and BT for each of the materials is supposed to be. ("H" is Hardness, "HP" is Hit Points, and "BT" is the Broken Threshold, and "FKA" is "formerly known as")

(6th) Iron (H 18, HP 72, BT 36), low-grade cold iron (H 18, HP 72, BT 36), standard-grade silver (H 14, HP 56, BT 28)

(7th) high-grade silver (H 20, HP 80, BT 40), standard-grade dawnsilver FKA mithral (H 18, HP 72, BT 36)

(8th) standard-grade cold iron (H 22, HP 88, BT 44)

(9th) high-grade dawnsilver FKA mithral (H 24, HP 96, BT 48)

(10th) high-grade cold iron (H 28, HP 112, BT 56), standard-grade adamantine (H 28, HP 112, BT 56)