Unfortunately, if it is current legal tender then the answer is 50p. However, if there is a genuine misprint (i.e. mistake by the mint, not a mistake during forgery) on the coin then it could be worth more to a collector.
If its an English 50p coin its worth almost nothing. At most you can buy a lollipop maybe a loaf of bread if your out in the country but to be honest you will hard pressed to find anything that cheap in the cities, especially London. Its not worth anything.
