Right now we have classic, 1GP=10SP=100CP,
but prices of those metals are not in that range,
An other poll about
the D&D economy preferred the two following values:
1 gold piece = $100
1 silver piece = $10
1 copper piece = $1
(38.7% of voters)
Alternatively:
1 gold piece = $10
1 silver piece = $1
1 copper piece = 10¢
(32.3% of voters)
Together, these two options represent 71% of the voters. The value of a gold piece is something like $100. Or precisely that depending on the standard size of the gold coin or gold armlet.
Heh, I hate to deal with pennies in reallife, and certainly dont want to deal with it fantasy! So, I myself strongly prefer the exchange rate of 1 copper piece = $1.
Additionally:
1 mithril piece = $1000
(For me, platinum feels too modern. Mithril is magically awesome. For mythological accuracy, adamantine − the ancient adamas − resembles a transparent sapphire crystal, and is actually an indestructable gem. Adamantine crystallizes irreversibly into a specific shape.)
Gold is ten times the value of silver: can happen. It can happen in certain premodern economies and in modern economies. Similarly, silver to copper. It is plausible. The round numbers are convenient. A gold piece is a $100 bill, a copper piece is a $1 bill. Use whichever bills make sense.
The point is, players can use todays reallife economy. For an actual realistic economy. If in realife, a high-quality handcrafted suit of full plate armor costs $3000, then it in gamesetting it costs 300 silver pieces or 30 gold pieces.
Money depends on the regional setting. But typically. The D&D economy is a magic economy. It isnt a medieval economy. The gamesetting uses magitech (magical technology) where we today use hitech to produce goods and services.
D&D players can participate in a realistic economy. How much money would you pay in reallife for a Potion of Healing? That is the price it costs in D&D in gold pieces. Depending on the setting, a Potion of Healing might be cutting-edge medical research, in which case it might cost hundreds of thousands of dollars. Or it might be more like a routine over-the-counter medication, in which case the Potions are $1 each. Either way, players can understand exactly why the item costs what it does. It is a realistic economy.