It's very difficult to give exact modern day equivalents of historical currency, but a piece of eight in the early 18th century had the buying power of roughly $50 and a British pound had the buying power of roughly $200 (Little, p. 251; Talty, p. 42). The average worker in the 18th century made only a few shillings a day (a shilling is 1/20 of a pound or about $10) but living costs were also a lot less back then so it's not a fair extrapolation to say the average worker had to be making the equivalent of even modern minimum wage. People made less, and things also cost less, just like in many poor countries today.

Going by this, 5 million pieces of eight is equivalent to about 250 million dollars. That's an extremely large fortune, but nowhere near a billion.

That also means 500 pieces of eight is the equivalent of about $25,000 today and we actually have contemporary accounts of pirates easily spending exactly this amount and more on alcohol and women in a single night if they were particularly reckless. The show Black Sails is obviously making a reference to this and it's both historically accurate and plausible to do so.

Here is an excerpt from the contemporary book The Buccaneers of America published in 1678 by an actual former buccaneer named Alexandre Exquemelin:

Captain Rock sailed for Jamaica with his prize, and lorded it there with his mates until all was gone. For that is the way with these buccaneers -- whenever they have hold of something, they don't keep it for long. They are busy dicing, whoring and drinking so long as they have anything to spend. Some of them will get through a good two or three thousand pieces of eight in a day -- and next day not have a shirt to their back. I have seen a man in Jamaica give 500 pieces of eight to a whore, just to see her naked. Yes, and many other impieties.

My own master often used to buy a butt of wine and set it in the middle of the street with the barrel-head knocked in, and stand barring the way. Every passer-by had to drink with him, or he'd have shot them dead with a gun he kept handy. Once he bought a cask of butter and threw the stuff at everyone who came by, bedaubing their clothes or their head, wherever best he could reach.

... Even the man I have just been speaking about, the one who gave the whore so much money to see her naked and at that time had a good 3,000 pieces of eight -- three months later he was sold for his debts, by a man in whose house he had spent most of his money.

To put this in more perspective, an adult male slave at this time was commonly valued by buccaneers at about 100 pieces of eight or about $5,000, but it also varied a lot. A musket could cost from 5 pieces of eight to 3 pounds or $250-600. A cutlass might cost 6 shillings or $60. Cattle could be bought at 4 or 5 pieces of eight or $200-250 a head. A gallon of rum commonly cost 4 shillings or about $40.

A very small ten-ton sloop or basically a sailboat could cost 30 pounds or about $6,000 and a huge 350 ton, 36 gun man-of-war frigate could cost 8,200 pounds or the equivalent of about 1.64 million today.

All the above price lists are from The Buccaneer's Realm by Benerson Little and The Republic of Pirates by Colin Woodard.

Sources:

Empire of Blue Water by Stephen Talty

The Buccaneer's Realm: Pirate Life on the Spanish Main, 1674-1688 by Benerson Little

The Republic of Pirates by Colin Woodard

The Buccaneers of America by Alexandre Exquemelin