Some people value websites at x10 --> x20 times the monthly income, but you've stated you don't earn anything from the site. There are also people who would be willing to pay a premium for your active members.
You can try this site for an very rough estimate of your site's worth.
dnScoop - Domain Name Value, History, Stats Tool and Forums
But as they say,
"it's worth the amount that people will pay for it"