Database sounds like a good idea. But when it grows the database needs lots of ressources (memory) which means money (as you said). So it would be tempting for the admins to monetarize it similar to Google, or?
A web directory already exists:
You could help expand it.
I like your idea. I wish I had a good idea how to start it small. Because, look at Wikipedia, it started as a small idea and look at it now!