Based SOLELY on the assumption that it is only the value of the material/metal that gives the various coins value (silver for silver, copper for copper, gold for gold etc.) and that the PHB tells us that one coin = 1/50 of one pound, the answer is quite simple.
A 50-gp ingot made of gold weighs one pound.
A 5-gp ingot made of gold weighs one-tenth of a pound.
And so on.
For US folks, someone mentioned that based on the density of gold, silver, and copper, a gold piece is about the size of a US dime, a silver piece is slightly larger than a quarter, and a copper piece is slightly larger than a silver piece.
If you want to tell me coins derive value thanks to rarity or government fiat, that's a different kettle of fish.
I know they do IRL, I am merely trying to make an assumption for fantasy worlds.
Now, to your REAL questions (just getting my assumptions out of the way):
1.) Given that iron is the main "ingredient" in steel, suppose it takes a Craft check to manufacture steel from iron. Since the market price for steel is 2/3 gp per pound (see #2 below) and the Craft skill indicates that creating such takes 1/3 the market price in materials (I assume the carbon needed is basically free since I assume charcoal's cost is negligible). Thus, iron is worth about 2/9 gp per pound (round to 2 sp/pound).
2.) A pound of steel - well, a dagger costs 2 gp, and it weighs one pound. Since the description of the Craft skill indicates that it takes 1/3 the market price in materials, we can conclude that steel is worth 2/3 gp per pound (which you can round off to about 6-7 sp/pound).
3.) A pound of mithral is worth the same as a pound of platinum (500 gp) per the DMG.
4.) A pound of adamantine (alloy) is worth 1700 gp (by my own guesses - see original thread here:
http://enworld.cyberstreet.com/showthread.php?s=&threadid=12885 ) but a pound of adamantium (pure metal) is worth 5000 gp.
5.) A pound of pigeon poo - I guess depends on whether or not you're selling it as a "material component" to a spellcaster.
--The Sigil
