How do I value a domain name?

I had big plans to create an eco-portal about 5 years ago. Unfortunately, I only got as far as registering the domain name :)

So someone has approached me to buy the domain name. I'm not a domain-squatting fan and would like to see it go to a good home. At the same time, I don't want to give it away then find it was sold on for more...

So here's what I have: domybit.co.uk domybit.com domybit.info domybit.net domybit.org

and I'm asking for £450 to cover the cost of the 4 years I've renewed them and a bit of time to sort out the transfer. What do you think they are worth? How do I value them? thanks hn.