The Galleon has numerals around the outside edge which is a serial number referring to the Goblin who cast the coin. It is unknown whether the Sickle or Knut also have these numbers, but it seems likely.
Hermione used a Protean Charm to turn fake Galleons into a secret communication tool for the DA. The numerals around the edge would change to indicate the date and time for the next meeting when Harry changed his (OP19).
For your convenience, a Wizarding World Currency Converter is available.
The actual values of these coins are a bit complicated for Muggles to figure out in Muggle terms without a calculator, and rather difficult for wizards when dealing with Muggle money (GF7). It's easy for wizards dealing only with wizarding currency, however. Hagrid states: "The gold ones are Galleons. Seventeen silver Sickles to a Galleon and twenty-nine Knuts to a Sickle, it's easy enough." Therefore 1 Galleon = 17 Sickles = 493 Knuts.
Well, even if you can do the math quickly in your head when you need to make change, it's still not easy. The values of the coins don't always seem to make sense. After all, you can buy a large quantity of sweets from the cart on the Hogwarts Express for eleven Sickles, about two-thirds of one Galleon. So is that all that the Weasleys had in their vault? The price of a pile of candy? And with that they bought books and school gear for all those kids? It just doesn't quite work out.
JKR has stated in an interview (CR) that she estimates the value of one Galleon to be "about five pounds," which works out to around US$9.75 (the exchange rate at the time of the interview was US$7.33). In the introduction to both QA and FB, US$250-million is stated to be the equivelent of 34 million Galleons. That also works out to a value of £5 to the Galleon, at the exchange rate of the time. The price listed on the back of the books, however, is not correct, since US$3.99 would equal less than half a British pound, or 8 sickles and 15 knuts. The book instead incorrectly lists US$3.99 as being equal to 14 sickles and 3 knuts. (Unfortunately, CNN.com uses this incorrect value for their Knuts-to-dollars converter.)
This gives us approximate values as follows:
There is apparently some kind of foreign wizard money that consists of gold coins the size of hubcaps (if Mr. Roberts wasn't simply giving an exaggerated description of Galleons) (GF7).
While wizard money seems to be made from actual precious metals, it also seems to have some sort of magic in it which makes it lighter than normal. Harry handled a bag containing one thousand Galleons-- the prize money from the Triwizard Tournament--as if it were nothing, but a thousand coins made out of gold, even fairly small ones, would weigh a considerable amount indeed, far more than anyone would be able to toss around in a cloth bag. This magic is perhaps an effect similar to Wizard Space.
amount Fred and George bet (their whole savings)
full set of second-hand Quidditch balls, slightly worn, Snitch having
a damaged wing
unicorn horn (PS5)
amount Hermione had set aside to buy herself a birthday present
amount Ludo said he'd pay for a rubber chicken wand
amount Arthur Weasley bet on the World Quidditch Cup match
(PS5, corrected edition)
pile of candy from the cart on the Hogwarts Express
black beetle eyes (PS5)
Hit-Witch or Hit-Wizard for the Magical Law Enforcement Squad
(new hire's starting salary, together with a
Ministry of Magic
broomstick and one's own regular
bed at St. Mungo's)
Assistant Manager at Flourish & Blotts Wizarding Bookstore
See also: Gringotts