Standard Exchange Rates, Expanded

 Here's some additional exchange rates designed to replace the chart in chapter 5 of the PHB, and the sources used.

These three sources establish the value of gold per pound and that the value of coin quantity to pure metal quantity is flat (there's no 'value added' by making the coin).

• Player Handbook, Chapter 5: "A standard coin weighs about a third of an ounce, so fifty coins weigh a pound."

• Player's Handbook, Chapter 5: "50 gp = 1 lb. of gold"

• Player's Handbook, Chapter 5: "... Rather, the gold piece is a standard measure of value, and the actual exchange is in gold bars..."

These sources establish the exchange rates for the metals not included in the original PHB chart (iron, mithral, and adamantine).

• Iron: Player's Handbook, Chapter 5: "1 sp = 1 lb. of iron". Dungeon Master's Guide, Chapter 1: "...iron is normally valued elsewhere (1 sp per pound)." Waterdeep: Dungeon of the Mad Mage, Level 21: "Sixty iron bars are stacked on the floor. Each bar is worth 1 gp and weighs 10 pounds."

• Adamantine: Waterdeep: Dragon Heist, Chapter 4: "The characters can purchase a 10-pound adamantine bar for 1,000 gp." (this sets the price of 1 pound of adamantine equal to 2 pounds of gold)

• Mithral: Waterdeep: Dungeon of the Mad Mage, Chapter 6: "One character can strip 1 pound of mithral in 15 minutes. Each pound of the metal is worth 50 gp." (this sets the price of 1 pound of mithral equal to 1 pound of gold)

The mithral and adamantine prices are also confirmed on this tweet, which unfortunately doesn't make it clear that he's talking about pounds.

Having said all that, Tomb of Annihilation, Chapter 2 has a very different price for adamantine. "Each adamantine ingot is worth 10 gp and weighs 1 pound." So here, 1 gp = 5 adamantine pieces (adamantine is significantly cheaper than gold in this section). It does say the ingots are "refined adamantine". I think its easy to explain this off as local market dynamics (a 'nonstandard' exchange rate) or the fact that these specific ingots aren't fully refined. Also note that Tomb of Annihilation doesn't have this regional pricing adjustment for iron as the exchange rate is that listed in the chart above "each iron ingot is worth 1 gp and weights 10 pounds).