You can use estibot.com as a help to get the stats and other informations which you can use to appraise the site.
Don't decide the value based on the automatic appraisal estibot gives. Only use the stats and other informations it gives.
Then try look at namebio.com to see how much similar domain names has been sold for.
I cant remember which thread, but there was a thread with good information and links to sites where you can get information you can use to base the value of the site. Try search here in Namepros.