Conversion reasoning

Scholastic has published two Harry Potter textbooks, written by J.K. Rowling. These books are Fantastic Beasts and Where to Find Them and Quidditch Through the Ages. On the back of these books is a pricetag that reads "$3.99 (14 Sickles, 3 knuts)". Armed with this information, one can use Algebra to figure out a conversion of knuts, sickles, and galleons to real world currencies.