Galleons

Galleons are fat gold coins used in the Wizarding World (PS5, HBP13), valued at approximately five British pounds (CR). The serial number identifying the Goblin who cast the coin is stamped around the edge (OP19). Coins of lesser denominations — silver Sickles and bronze Knuts — are also in circulation (PS5).

Harry had inherited a lot of galleons from his mother and father (PS5), a Potter ancestor having invented many useful potions including Sleekeazy’s Hair Potion, and added to his treasure when he inherited Sirius’ money as well (HBP3).

The Galleon was worth considerably more in the 13th century. When Barberus Bragge offered a 150 Galleon prize to the player who caught a snidget bird during a Quidditch match in 1269, the value of that prize is given as over a million Galleons in today’s currency:

1. Equivalent to over a million Galleons today. Whether Chief Bragge intended to pay or not is a moot point. (footnote in QA4)

That means that the value of one Galleon in 1269 is the equivalent of over 6666 Galleons today (US$48,333). At that rate, one Sickle would be worth 392 pounds (US$2842) and a Knut would be worth around 13.5 pounds (US$98). This obviously makes little sense in today’s terms, which suggests that the relative values (17 Sickles to the Galleon etc.) might not have held true 800 years ago.

While Galleons are described as fat and heavy, they are not as heavy as they would be were they actually solid gold, judging by the fact that a thousand of them in a bag was carried in a pocket (GF36). This could be a magical effect or that the actual gold content of a Galleon coin is considerably less than 100 percent.

NOTE: The current exchange rate for a Galleon can be found on the Lexicon’s home page. The Gringotts Goblins have taken the currency converter offline for now until they can get it working properly… and until the Ministry of Magic comes up with the Goblins’ 10,000 Galleon fee for doing the work.

• 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.
• 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.
• Arthur Weasley won 700 Galleons from the Daily Prophet Draw and took his family to Egypt, but they kept back enough to buy Ron a new wand (his old one was damaged while crashing the flying car into the Whomping Willow) (PA1).
• Hermione spent 10 galleons or less on her cat Crookshanks in Diagon Alley (PA4)
• The Weasley Twins made a bet with Ludo Bagman on the Quidditch World Cup with all the money they had -- "thirty-seven Galleons, fifteen Sickles, three Knuts" (GF7) They won the bet that Ireland would win but Viktor Krum would catch the Snitch, but Bagman first balked at paying them, then paid them with leprechaun gold that disappeared, then refused to give their money back, saying they were too young to gamble anyway. After hearing the sad story, Harry gave them all his winnings from the Triwizard Tournament, over 1000 Galleons, and they  used it to start their business Weasleys Wizard Wheezes (GF37).
• Hermione enchanted fake Galleons for Dumbledore's Army that would show the date and time of meetings, and would grow hot when the date or time changed. She used a Protean Charm so that when she changed her own, everyone's would change (OP19).
• Rowling, when asked the value of a Galleon, responded that it was worth "about five pounds" (CR). This is the value used to calculate the exchange rate found on the Lexicon's home page.
• In 1926 New York at the speakeasy known as The Blind Pig, Newt Scamander offered the Goblin Gnarlak a "couple of Galleons" for information about what was going on, but instead he preferred Newt's Bowtruckle Pickett for his more valuable lock-picking skills (WFT).