The value, like with anything else, is simply what someone will give you for it. It's psychology. There is no hard calculation you can do to arrive at the 'value' of a bitcoin.

The trust between the nodes and the network may be a factor that informs some people. Some people will use it because they consider it to be free from central control, or anonymous, or because "Hey! Computer Money!", or simply because they have a gut feeling it's the next big thing.

HTH.