A sixpence is the equivalent of about 50 cents in 2014 dollars (28.8 pence in British terms). However, the British have not used sixpence for currency since the early 1970s.
In 1971, the British switched over to a currency system based on tens. Prior to the switch, a pound had 20 shillings in it. A shilling was 12 pence (what the United States considers pennies). A sixpence was half a shilling, so as the name implies, it represented six pence. In other words, a pound had 240 pence in it. In the current system, a pound has 100 pence in it, just as a U.S. dollar has 100 pennies.