They are worth what someone will pay for it, cases on their own come up so infrequently that there is no yard stick for them. Often people who come across a case will seek the razor to match the case and make the set up. Some people will even buy a mismatched auction set just for the case and sell the razor that came with it on it's own.
I personally would not pay more then a few quid for a case unless it was in mint condition and was one I really really wanted, even then I would not pay a great amount, but someone might. Stick it on the bay if you feel lucky.