The largest unit of money in the wizards' world is a gold coin called a Galleon. One Galleon is worth 17 Sickles, and one Sickle is worth 29 Knuts, thus, 1 Galleon = 493 Knuts. Oddly enough, it does not seem as though there are any coins other than these three.
In the book Fantastic Beasts And Where To Find Them, it is said that £174 million raised for charity is equivalent to 34,000,872 Galleons, 14 Sickles, and 7 Knuts (the figure is simplified to 34,000,000 Galleons in Quidditch Through the Ages). This indicates a value of £5.12 for a Galleon, a Sickle is worth roughly 30.1 pence, and a Knut is worth about 1.04 penny.
At the current exchange rate, this works out in American money to approximately $9.13 per Galleon, 53.7 cents per Sickle, and 1.85 cents per Knut.
This suggests that a Galleon is not actually gold, or at least, not entirely gold, as 5 pounds worth of gold would make for a very small coin.