When is a penny worth $300,000?

When it's one of the first copper coins struck at the U.S. Mint in Philadelphia, in 1793.

The annual Florida United Numismatists (FUN) convention is being held in Tampa this week, and three historic coins, including the pricey penny, went up for auction Thursday evening.

A copper penny struck at the U.S. Mint in Philadelphia in 1793 sold for $300,000 at auction on Thursday

The annual Florida United Numismatists (FUN) convention is being held in Tampa this week

Other high-value U.S. coins sold at auction included a half-dollar minted in New Orleans in 1838 and a $4 gold piece called a 'Flowing Hair Stella,' (pictured) sold for $750,000

Mark Borckardt, a senior numismatist with Heritage Auctions in Dallas, said the George Washington-era penny is one of about 500 in existence.

The 63rd annual FUN Convention also features 'over 600 dealer booths, a competitive exhibit area, 15 educational programs' and a host of other activities. FUN describes itself as 'an organization dedicated to involving and growing the numismatic community.'

Other high-value U.S. coins sold at auction included a half-dollar minted in New Orleans in 1838 and a $4 gold piece called a 'Flowing Hair Stella,' that sold for $750,000.

The 1838 half-dollar one of the rarest and most enigmatic coins in U.S., according to Hermitage Auctions, and 'is believed to be the earliest branch mint proof coin of any denomination.' HA adds that no official record of its mintage exists.

Heritage Auctions shows a half-dollar minted in New Orleans in 1838 that went up for auction at the annual Florida Numismatic Convention in Tampa, Fla., this week

The coin is one of the rarest and most enigmatic coins in U.S. history and only 10 examples are known to exist in the world.

It remains unclear how much the half-dollar sold at auction.

According to the Professional Coin Grading Service (PCGS), the leading US authority on coin authentication, the 'value of a coin is based on rarity and condition, not how old it is.'

The industry saw record numbers last year, with the Heritage Auctions website stating that 'high-profile sales in World and Ancient coins' reached $49 million in sales in 2017.

'The growth in World & Ancient Coins is significant and is energizing Heritage's global expansion,' said Cristiano Bierrenbach, Executive Vice President of Heritage Auctions.

'Offices in Hong Kong, Amsterdam and our latest office in London are all expanding the level of service we can provide clients in this increasingly diverse marketplace.'

Last year also saw a rare 1870 Japanese Meiji gold Proof sold for '$470,000, against a $100,000 estimate.'

Last year also saw a rare 1870 Japanese Meiji gold Proof sold for '$470,000, against a $100,000 estimate'