My guess is that the money should not come from the users nor the ads Then where could the money possibly come from? Twitter struggled with this in the beginning, and one idea they tossed around was charging outside companies for analytics. It doesn't sound like you have enough traffic for that. You may have a site that is just not monetizable.
It would probably have to be an idea to add value for a business (since the normal ads and user won't bring in the real money). Maybe you are right about the monetizable, Hacker News was kind of my last hope to get to that idea. :)