To be honest its very difficult to get an accurate appraisal. Its very much like domains in that respect. Whatever someone is willing to pay for it is the short answer but thats not much help...
Heres a short summary of what you need to consider.
http://tidget.com/help.php?action=showitem&id=1
and heres a really cool tool by the same guy that might help. Ya its a script, but from all reports its not too bad...The script is only in beta though.
http://tidget.com/index.php?custom=yes&TID=custom-tools