Canon:Gold piece

A gold piece, or gp for short, is the most common coin in D&D worlds. It is worth ten silver pieces or a hundred copper pieces. 10 gp equals 1 pp. The standard coin is supposed to weigh about a third of an ounce (fifty to the pound). On average, having 1 gp, one can buy a goat or a pound of cinnamon. The most powerful of magic items can cost 1'000'000 gp and more.

Roman gold coins
Gold was used for coinage very infrequently until the time of Julius Caesar, who introduced a standardised coin called aureus, which was struck regularly. It weighed 8 gramm, 1/40 of a Roman pound, but later its weight decreased to 1/45 of a pound in the time of Nero and to 1/50 of a pound in the time of Caracalla. The aureus had a fixed value of 25 denarii (Roman silver coin). Emperor Constantine I introduced the solidus to replace the aureus. Solidi were wider and thinner than the aureus, with the exception of some dumpy issues from the Byzantine Empire. The weight and fineness of the solidus remained relatively constant throughout its long production, with few exceptions. Fractions of the solidus known as semissis (half-solidi) and tremissis (one-third solidi) were also produced.

British gold coins
The noble was the first English gold coin produced in quantity, having been preceded by the twenty pence coin and the florin (also called double leopard) earlier in the reigns of King Henry III and King Edward III, which saw little circulation. The coin was introduced during the second coinage (1344-1346) of King Edward III, when the coin weighed 138.5 grains (9.0 grams); during the kings' third coinage (1346-1351) the weight of the coin was reduced to 128.5 grains (8.3 grams), while in his fourth coinage (1351-1377) it became even lighter, at 120 grains (7.8 grams).

A gold sovereign is a gold coin first issued in 1489 for Henry VII of England and still in production as of 2006 (equal to a pound sterling). Those original sovereigns were 23 carat (96%) gold and weighed 240 grains or one-half of a troy ounce (15.6 grams). Henry VIII reduced the purity to 22 carats (92%), which eventually became the standard; the weight of the sovereign was repeatedly lowered until when it was revived after the Great Recoinage law of 1816, the gold content was fixed at the present 113 grains (7.32 g), equivalent to 0.2354 Troy ounces. Sovereigns were discontinued after 1604, being replaced by unites, and later by laurels. Production of sovereigns restarted in 1817.

The guinea coin of 1663 was the first British machine-struck gold coin. The coin was originally worth one pound, which was twenty shillings; but rises in the price of gold caused the value of the guinea to increase, at times as high as thirty shillings. 44½ guineas would be made from one Troy pound of 11/12 finest gold, each weighing 129.4 grains. In 1670 the weight of the coin was reduced from 8.4–8.5 g to 8.3–8.4 g, but the price of gold continued to increase, and by the 1680s the coin was worth 22 shillings. The diameter of the coin was 25 millimetres throughout Charles II's reign, and the average gold content (from an assay done in 1773) was 0.9100.