Successful Directories ??

Hi guys – I’ve been looking through the showcase section of this forum. I’m wondering if there are any clear examples of successful web directories powered by Directory Press?

What I’m really looking for is some form of guide on how to pull off a successful PAID business directory.

I’ve got some ideas in my head but would love to hear tactics/process that’s maybe been used to build something that generates over $10,000/month.