Money in the Wizarding World is quite different from that in the Mundane world. In the books, Rowling cites that a single galleon is roughly worthy 5 pounds sterling (5 quid, or a fiver to those who enjoy slang). In 1976, the dollar to pound exchange rate was quite different and actually was facing a crisis that forced the UK to borrow money, all as the result information being leaked from the International Monetary Fund, who felt the pound was only worth $1.50 at the time. Since then the exchange rate at lowest was 1 pound=1.05 dollars in 1985, and today stands at roughly 1 pound = $1.91.

That said, assuming that the 1 galleon=5 pound exchange rate is actually stable over time (and well, why shouldn't it be, wizards are very stable people!) here's what the values were roughly in 1976, when the pound was worth $1.57 in dollars:

* 1 galleon = 5 pounds = $ 7.85
* 17 sickles = 5 pounds = $ 7.85
* 493 knuts = 5 pounds = $ 7.85

These values are calculated by the HP Lexicon here.

Information on Exchange Rates was taken from here.