The Player's Handbook lists some commodities on Table 7–3: Trade Goods (112), including iron at 2 sp per lb. (therefore 6 sp for 3 lbs., although it's unmentioned if the iron is refined or unrefined). The Dungeon Master's Guide lists an item made of mithral as costing an additional 500 gp per lb. of the original item (leading one to believe 50 gp of mithral weighs 1/10 lb.).
The Arms and Equipment Guide also has a list of trade goods and corresponding values (40).