I'm a bit bothered about the size of the "standard" coinage for D&D, especially at the 50 coins = 1 pound exchange. I'm thinking of using a 500 coins = 1 pound exchange as a possibility (BTW, what would the coin dimensions be for that?).
In the interest of "real" coinage, here's a few things to ponder:
Debasement: Gold & silver, in their pure form, are way too soft for coins. They'll most likely be alloys for durability's sake. However, if a coin gets too debased (basically falling under 75% of the "main/name" metal in the alloy), then the coin's not going to have that much value at all (and may be harder to detect counterfeits).
Make a new "penny": Use another metal (such as zinc, tin, lead, iron, aluminum, etc.) as the lowest-value coin in the game, and bump copper to the status of a silver piece, silver to a gold, etc. Gold will be rarer, & platinum even more so.
Some other metals used for coinage:
Zinc
Tin
Lead
Iron
Nickel (found in deposits of platinum & palladium)
Aluminum (the mining/refinement process for this metal may not be feasible for a medieval culture to use this)
Alloys, alloys, alloys: Along the ideas of debasement, as well as a new "penny," but certain metal-mixtures may work for common currency, lower-value currency, etc.
Billon: a copper-silver alloy in which there is a higher content of copper than silver.
Red gold: gold alloyed with copper
Electrum: the famous gold & silver alloy
Whitegold: gold alloyed with either platinum or palladium (IIRC, IRL palladium is worth more than gold, but less than platinum)
Brass, Bronze: common copper alloys
Pewter: a tin & lead alloy
There are many more alloys, but those are a few that I know of offhand.
IMHO, I'd think that materials that may be needed for tools & weapons (like iron/steel) wouldn't be the best metal for coins, since there'd be temptation to melt down the coins for other uses (unless that's a cool idea for you, but that brings in counterfeiting issues). Metals that can severely corrode (like iron & tin) wouldn't be good choices for coinage either.
Here's a possible coinage system, still using a sort of decimal system for the values:
Pewter piece: $0.001
Zinc piece: $0.01
Brass piece: $0.05
Nickel piece: $0.10
Copper piece: $0.25
Billon piece: $1
Silver piece: $5
Redgold piece: $10
Electrum piece: $20
Palladium piece: $50
Whitegold piece: $100
Gold piece: $200
Platinum piece: $500
Of course, this is WAY too many coins for one system--but you can sort of get an idea.
FYI, here's some info I found on values for precious metals & the like:
1 ounce Gold = 72.3589 ounces Silver
1 ounce Gold = 2.06027 ounces Palladium
1 ounce Platinum = 2.01751 ounces Gold
This site provides conversion info. Click on "Money," then select the denominations/values of your choice.
However, this is all going on the idea that the rarity of these metals in the specific fantasy setting matches the levels of rarity for the metals on Earth. Gold and silver could be more common in those worlds, & thus their values as coins reflect this.