What it�s worth is how much someone will be prepared to pay for it. Obviously it depends on condition and how much MoT left etc.
If you�re thinking of selling it, a dealer would only offer you the �book� price. Fiestas can sell very well on ebay (even those without any MoT seem to go for remarkably high prices sometimes). I know this because I�ve tried to buy a good old one there, as they�re reliable cars and cheap to run.