To know the value of a domain name, one has to use an appraisal service. But it does not come cheap when looking for accurate tailored service. Here are some sites that offer appraisal for free.

1. www.nameboy.com - My favorite so far. Create a dedicated report for your domain and explain the criteria.
2. www.overcent.com
3. www.dnscoop.com/
4. www.leapfish.com  - seems to be down