I have found many references to copper being extremely valuable in the 17th century, most notably stories surrounding Falu Koppargruva in 1630s Sweden. I have however been unable to find any reference to what the copper was actually used for.
Copper was very commonly used in coinage. For most basic everyday transactions, silver and especially gold coins were too high in value to be used. Copper was thus used in low-value coins throughout Europe. There were certainly other uses as well (such as casting bronze cannon), but this was the main one.