In the first book, Harry sees his Gringotts vault for the first time.
(Quotes thanks to Slytherincess!)
Griphook unlocked the door. A lot of green smoke came billowing out, and as it cleared, Harry gasped. Inside were mounds of gold coins. Columns of silver. Heaps of little bronze Knuts.
"All yours," smiled Hagrid.
All Harry's -- it was incredible. The Dursleys couldn't have known about this or they'd have had it from him faster than blinking. How often had they complained how much Harry cost them to keep? And all the time there had been a small fortune belonging to him, buried deep under London.
Hagrid helped Harry pile some of it into a bag. "The gold ones are Galleons," he explained. "Seventeen silver Sickles to a Galleon and twenty-nine Knuts to a Sickle, it's easy enough. Right, that should be enough fer a couple o' terms, we'll keep the rest safe for yeh."
Harry Potter and the Sorcerer's Stone - pages 58-59 - US Hardcover Edition
Jk Rowling posts in the Harry Potter Lexicon
JKR has stated in an interview (Comic Relief 2001) 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:
1 bronze Knut = £0.01 (US about 2 cents)
1 silver Sickle = £0.29 (about US$0.57)
1 Galleon = £5.00 (more or less US$9.75)
These rates vary as the exchange rate fluctuates - see the Wizarding World Currency Converter for the current rate.
Now looking at the quote
Hagrid helped Harry pile some of it into a bag.
Right, that should be enough fer a couple o' terms, we'll keep the rest safe for yeh.
Obviously "some" is unspecified, but presuming the costs of school textbooks...
Typical elementary-school textbooks cost more than $100 each
He would be carrying at least 10 galleons for each school book, and going by the list of books just for his first year,
All students should have a copy of each of the following:
- The Standard Book of Spells (Grade 1) by Miranda Goshawk
- A History of Magic by Bathilda Bagshot
- Magical Theory by Adalbert Waffling
- A Beginners’ Guide to Transfiguration by Emeric Switch
- One Thousand Magical Herbs and Fungi by Phyllida Spore
- Magical Drafts and Potions by Arsenius Jigger
- Fantastic Beasts and Where to Find Them by Newt Scamander
- The Dark Forces: A Guide to Self-Protection by Quentin Trimble
(Harry Potter and Philospher's Stone, Chapter 5)
That's 80 Galleons just for books. This does not include the list of other expenses Harry has for his school years (wand, potion reagents, etc.)
How many Galleons is "some" or is Diagon Alley selling supplies for third world prices?
Sidenote, is this just a case of bad JK maths?