Originally a gold coin, the guinea is still referenced as a set amount of money. I came across the following information that explains a bit about it.

LONDON – “That will be eight guineas, moddom,” said a clerk in the Burlington Arcade as she slipped a cashmere sweater into a black bag.

I handed her 8 pounds (each worth $2.80) and 8 shillings (each worth 14 cents). There was not a guinea in sight.

“Two guineas, if you please,” said the clerk in the man’s shop on fashionable Bond Street. He was knotting a fine maroon colored box bearing the firm’s gold lettering and sheltering the silk tie I had just bought.

I counted out 2 pounds and 2 shillings and went on my way. More of the invisible guineas went for a jacket at Fortnum’s, the specialty store on Piccadilly with the grocery section manned by clerks in black frock coats. Still more guineas were asked for each of several large books from Matchard’s, next door.

“Do you know,” I remarked to a friend from the royal mint, during much of this morning while shopping I have been paying prices quoted in terms of a gold coin, the guinea, that ceased to be minted more than a century and a half ago? What do you think of that?”

“I suppose there’s a certain snobbish enjoyment of the old term,” he replied. “The gold coin first appeared in King Charles II’s time, in 1663, gaining its name from the gold’s source — the Guinea coast of Africa. It was a nice coin, about the size of your quarter but thin, and worth 21 shillings. In 1817 the Guinea gave way to the gold sovereign, now also out of circulation, which was worth only 20 shillings because during the Napoleonic wars people had become used to paper bank notes and counting in 20 shillings, or one pound.

“Fees for doctors and lawyers, as well as prices for fine goods like paintings, race horses, and certain exclusive clothes and specialty items, however, continued to be priced in guineas. They usually still are.”

“We don’t hear people quoting prices in groats and farthings,” I remarked, “and these coins, too, are no longer minted,”

“Ah, yes,” he replied, “but groats and farthings declined to purchasing value until they bought nearly nothing. That’s the way they disappeared. However, when the golden Guinea ceased it still represented one shilling more than one pound. I think that’s the crucial point. The guinea today remains an elegant way of getting more money out of the customer. What to charge? Eight pounds 8 shillings? Or eight guineas?Guineas, of course.”

So that’s why the reference to the Gold Coin Guinea, still lingers on.