One crown is the equivalent of one hundred coppers. In the book The Last Wish (Polish: Ostatnie życzenie) Nenneke tells Geralt, that the Crown currently has a value of six and a half orens and is still rising.
The Crown is the main currency in The Witcher 3. Other currencies, like orens and florens can be found throughout the game, but they cannot be used to purchase items from merchants. They can only be brought to Vimme Vivaldi at Novigrad's Vivaldi Bank and exchanged for crowns.