You should really tell us what coin it is first. In my experience each 'extremely rare' coin is different. Some are worth just a couple of hundred pounds, some are worth a couple of thousand, some more. It depends on what it is, and like you say, the demand for whatever denomination it is. In my book, there are not a lot of occurences of 'extremely rare' and where there are 'extremely rare's they usually concern coins that are highly unlikely to be out there in the hands of the public.