Not sure how true this is,
but the story is that only 500 were made and accidently went out, (this was when they were flipping the coin to print the date on the reverse. But 500 never printed by accident).
They went into circulation and now are worth a mint.
My bf has a friend who sold his on ebay for about £500 he said.
Now don't take my word for it, but this is the story that i heard 3 times from different people.