D'Urfey is using Guinea referring to the coin, clearly enough. But he could have been making some kind of pun too (though I don't think he was), since "a guinea" has often enough been used to mean a black person. The word refers to the West Africa, where a lot of gold came from, both metal gold and "black gold" which was a term for slaves.

In modern times the guinea wasn't a coin, but was regularly used as a measure of price, right up until decimalisation and the end of the old currency. A guinea was 21 shillings, or one pound and one shilling. (The style of currency you only get in Harry Potter these days, unfortunately.)