The measures are a little confusing, because while gold and silver are priced in troy ounces, copper is priced in standard avoirdupois pounds. But in troy, avoirdupois, and apothecaries measures, a grain is a grain is a grain. One troy ounce is 480 grains and one avoirdupois pound is 7000 grains.
Since in D&D there are 50 coins to the pound, each coin weighs 140 grains. That's very slightly more than 9.07 grams, or just less than 1/3 of an ounce (for comparison, a Sacagawea dollar weighs 8.1 grams; four dimes are about 9 g). Get the spot prices for gold, silver, and copper, from Kitco.com, and plug it all into a spreadsheet. Official coinage is somewhat more valuable than scrap and bullion, because it carries official authenticity, has numismatic value, and is scarcer. But the spot price sets its intrinsic value.
Last Friday (February 27, 2009), gold cost $939.60/oz, silver cost $13.23/oz, and copper cost $1.51/lb. So a d20 gold piece is worth $274.05, a silver piece is worth $3.86, and a copper $0.03.
Today's levels are the result of a decade-long runup in commodity prices, which persists for precious metals; older prices are a little more typical. On June 1, 2001, gold was $266.70/oz; silver was $4.58/oz; and copper was $0.745/lb. This means that a d20 gold piece was worth $77.79, a silver piece was worth $1.28, and a copper piece was worth a penny.
Since the d20 system uses standard equipment prices, it's simple to compare game prices to contemporary real ones. A longsword costs 15 gp, or $4,110.75 in today's money; a +1 longsword costs more than a quarter million dollars. A 2-gp backpack costs $548.10, a vial of ink costs $2,192.40, a fishhook costs $3.86, and a day's worth of trail rations is $19.29. A donkey costs as much as a vial of ink, a light riding horse costs $20,553.75, and a pig is worth about eight hundred dollars. An ordinary, skilled 1st-level commoner earns nearly $30k/year. Raising someone from the dead, a spellcasting service too expensive to be generally available, would cost at least $1.49 million, while removing a negative level would merely cost about a hundred thousand dollars.
The currency of Rome, by contrast, used a multi-metal standard. After the currency reforms of Augustus Caesar, the gold aureus was worth 25 silver denarii, 100 bronze sestercii, or 400 copper ases. The bronze sestercii weighed about 25-28 g. The silver denarius weighed about 3.9 g (about $1.66 of silver), and served as the backbone of Roman finance. Roman prices and currency varied greatly.
A century ago, the US used a silver dollar, a coin with 24.057 g actual silver weight. That specie is worth $10.23 in today's money, but an inflation calculator will show that a 1909 dollar is can buy $22.81 worth of goods in 2007 dollars. The word dollar comes from the Austrian "thaler," a Bohemian coin originally minted in 1518. The actual American dollar comes from the Spanish reales de ocho, the piece of eight, which Spain had shipped by the galleon-full from its rich silver mines in Mexico and Peru. At the time of the American Revolution, the states had no currency of their own, and approved the pieces of eight for American commerce until a dollar with the same value could be established. The eight reales coin was often cut into eight bits for change: two bits was thus equivalent to a quarter dollar. The New York Stock Exchange traded in eighths and sixteenths of a dollar until as late as 2001.
Of course, comparing prices between historical periods, much less fantasy worlds, is misleading. Agatha Christie expected to never be rich enough to buy a car, but never too poor to be without servants. And mass production and economic development has made things much, much more affordable, as Brad DeLong notes:
Our goods are not only plentiful but cheap. I am a book addict. Yet even I am fighting hard to spend as great a share of my income on books as Adam Smith did in his day. Back on March 9, 1776 Adam Smith's Inquiry into the Nature and Causes of the Wealth of Nations went on sale for the price of 1.8 pounds sterling at a time when the median family made perhaps 30 pounds a year. That one book (admittedly a big book and an expensive one) cost six percent of the median family's annual income. In the United States today, median family income is $50,000 a year and Smith's Wealth of Nations costs $7.95 at Amazon (in the Bantam Classics edition). The 18th Century British family could buy 17 copies of the Wealth of Nations out of its annual income. The American family in 2009 can buy 6,000 copies: a multiplication factor of 350.
Books are not an exceptional category. Today, buttermilk-fried petrale sole with pickled vegetables and parsley mayonnaise, served at Chez Panisse Café, costs the same share of a day-laborer's earnings as the raw ingredients for two big bowls of oatmeal did in the 18th Century. ...Today we still spend about one dollar in five on food—down from the half of income that Americans spent in 1776. The share hasn't fallen more because some of us buy buttermilk-fried petrale sole with pickled vegetables and parsley mayonnaise cooked, served, and cleaned up by others rather than (or in addition to) oats in the gunnysack. One reason is that the oats-for-five-meals-out-of-six-diet of 18th Century Scots was monotonous, and we are glad to escape it.