There are three universal coins, which apply to all areas of the world.

Copper coins, which are a mixture of copper, nickel and silver, which weigh nominally half an ounce (15 grams).
Silver coins, which are a mixture of silver and nickel, which weigh nominally a third of an ounce (10 grams).
Gold coins, which are a mixture of gold, nickel and silver, which weigh nominally a quarter of an ounce (8 grams).

12 c.p. = 1 s.p.
16 s.p. = 1 g.p.
192 c.p. = 1 g.p.
1 g.p. = 1 experience point (when seized in battle)

The weights above only apply to circumstances surrounding the trade tables, within which the weight of the metal itself has never been satisfactorily reconciled. Therefore at this time - barring inspiration - the weights of the coin are immaterial for game matters. In effect, all coins are considered to weigh one third of an ounce.

Limitations upon the number of coins which may be carried in a sack, pouch, chest, moneybelt and so on are based on volume, not upon weight. Coin containers listed in the equipment lists are as follows:

backpack - 3,700 coins
belt pouch, large - 250 coins
belt pouch, small - 80 coins
chest, large, 18 in. high, 18 in. long, 12 in. deep - 32,000 coins
chest, small, 18 in. high, 18 in. long, 12 in. deep - 8,000 coins
money belt - 450 coins
sack, 20 inches deep - 1,280 coins
saddle bags, large - 3,900 coins
saddle bags, small - 1,920 coins

Yes, I give experience points for gold, silver and copper coins.