The prospect of unexpectedly coming into riches is undoubtedly alluring, but how does it actually happen unless you win the lottery?

It turns out the answer could be in your wallet. Or pocket. Or stuffed down the back of your sofa.

You may know that the design of 50p coins affects their value - the most rare one is being sold for up to £220 on ebay, The Sun reports.

(Change Checker (Change Checker)

The Kew Gardens design 50p piece has been going for hundreds of times its cost on ebay and is the most sought-after 50p in circulation, with a mintage of just 210,000.

A ‘scarcity index’ has been created by Change Checker which tracks which circulating 50p coins are the most scarce and collectable.

After the Kew Gardens design, the next four most valuable coins are from the 2012 Olympics - football, triathlon, judo and wrestling.

They’ve been selling for up to £15 on ebay, so could still earn you a pretty penny.

The sixth most valuable 50p coin is the Beatrix Potter design, followed by Jemima Puddle-Duck in seventh place.

Change Checker explained how they calculated the index:

“Generally collectors have had to rely upon mintage figures to identify the scarcest coins. But they only tell part of the story. Trying to find a good quality coin from 15 - 20 years ago, even for a higher mintage issue, is much more challenging than a more recent issue, as coins become damaged over time and are ultimately removed from circulation.”

They point out that some designs are more popular than others too and thus hoarded by people who wouldn’t normally collect coins - Beatrix Potter is one such design.

It often takes the Royal Mint a while to confirm mintage too.

“That’s why we have combined the mintage information with two other key pieces of information,” Change Checker explains.

These are:

How many of each design are listed as ‘collected’ by Change Checkers, indicating the relative ease of finding a particular coin. The number of times a design has been requested as a swap over the previous 3 months, showing the current level of collector demand.