Do you have a 50p coin worth £75? Check your change now

The spare change in your pocket could be worth a pretty penny, with one rare 50p coin currently priced atup to 150 times its face value.

The 2009 Kew Gardens coin, released in 2011 to celebrate the 250^th anniversary of the Royal Botanical Gardens, is currently being sold on Ebayfor around £75.

But this isn't the only bit of changeto attract a premium on the collectors' market. Find out why these coins are so valuable and how you can spot one.

Why makes a coin more valuable?

Some coins are highly sought-after by collectors due to their scarcity, either because a limited batch was minted or because of their age. Only around 210,000 Kew Gardens coins are currently in circulation - around 0.02% of the 50bn 50p coins circulating overall.

The rare Kew Gardens coin was designed by Royal Academy President Christopher Le Brun and features the famous Chinese pagoda at Kew.Yet the coin's materials are the same asa regular 50p piece - 75% copper and 25% nickel - and it has the same diameter, weight and thickness.

Another hard-to-find 50p design is a 2003 release celebrating the suffragette movement, as well as a 2011 coin decorated with animals to honour the World Wide Fund for nature.

The table belowfrom coin experts Change Checker (changechecker.org) shows how rare certain 50p designs are in the British market.

Other factors that influence coin value

Apart from scarcity, a coin's value will also be determined by demand from collectors. Change Checker values coins by considering how easy it is to find a certain design in circulation, how many were minted, and how often the coin is traded between collectors.

In addition, prospective buyers will take into account the coin's current condition, including how tarnished or worn the surfaces are. While some collectors only pursue coins that have been in circulation, others prize those which were never released publicly - known as 'mint' condition.

Especially in-demand 50p coins include a seriesfeaturing Beatrix Potter characters such as Jemima Puddleduck and Squirrel Nutkin, which go for about £6 each. Other valuable designs include London 2012 coins featuring sports such as football and swimming.

What if I find a valuable coin?

If you think you've struck gold in your money box, the first thing to do isdouble-checkwith the Royal Mint if the coin is real or fake. They usually can provide a letter to confirm.

Once you know the verdict, you may be able to sell it at auction, online or via a coin dealer.