Complex sentence: Coins were at the time valued by their precious metal content, and the price of gold relative to silver rose soon after the guinea's issuance, and it came to trade at 21 shillings or even sixpence more.
Split into simpler sentences: Coins were at the time valued by their precious metal content, and the price of gold relative to silver rose soon after the guinea's issuance. Thus, it came to trade at 21 shillings or even sixpence more.