There are five basic metal types that are used for coinage.
Copper 10 cp = 1 sp
Silver 20 sp = 1 gp
Electrum 2 ep = 1 gp
Gold, the standard
Platinum 1pp = 5 gp
Thus, 200 cp = 20 sp = 2 ep = 1 gp = 1/5 pp
Certain metals aren't generally used for currency, but have an established value relative to gold:
Mithril - 1 mithril piece = 25 gp
Adamant - 1 adamant = 125 gp
Meteorite - 1 meteorite = 625 gp
When trying to figure the cost or value of mundane items, you can usually assume 1 gold piece is about $20. This really only works for inexpensive items; a real-world race horse would cost far more than the $800 you might extrapolate from 200 gold on the gear price list. Some things might be somewhat cheaper or more expensive.
Coins weigh about 1/10 of a pound, making gold and platinum fairly small, and copper and silver quite large. Density values: platinum 21,4 g/cm3; gold 19,3 g/cm3; silver 10,5 g/cm3; copper 8,9 g/cm3.
This means, 1m3 of gold or platinum weighs about 20 tons, 1m3 of silver or copper about 10 tons.