Something's value is how much
someone will pay for it. It doesn't matter how much you think it's
worth, or how much you put into it, if someone won't buy it, then its value is
zero. There are other ways of determining value in the real world, like how much something means to you, etc., but pure
monetary value is determined, in the end by how much someone will pay for it.
Ebay is a great forum for determining something's value.