Depends what you're talking about. What determines rarity is a combination of mintage, popularity, survival, etc. For example, Maundy sets are minted each year - around 1000 or so - which for a currency coin would be very rare. But because it's "only" Maundy, they fetch quite modest sums (there are many fewer collectors for them). The number of "undated error" 2008 20 pence pieces far exceeds Maundy, but look what they are selling for on eBay. A rough rule of thumb for modern currency coins would be that any mintage of less than a million makes it pretty scarce. But condition matters too : 1950s half crowns and florins were minted in large quantities but are often very hard to find in top grade, and can be worth up to £50 in mint condition, but nothing in ordinary worn condition. It's a more complicated subject than your question suggests. That means a Brittania made in 2007 is rare (only 700,000 exist)maybe a few 2 pounds