Can everyone please agree that a directory has no value unless it's actively maintained? Otherwise it's simply a list of sites that were useful in 1998. Users don't see the contract that says "You are responsible for keeping your information up to date". They only see the bad links.
This is assuming for the sake of discussion that the directory has human users. If it doesn't, it's obviously worthless anyway.
Once you've got the requirement for active ongoing maintenance, a one-time inclusion fee simply makes no sense as a business model. Who does this? It's initial inclusion fee maybe, plus periodic maintenance fee definitely. I was trying to think of real-life businesses that provide permanent service in exchange for a one-time fee. The only thing I could come up with was gravesite maintenance. Urk. There must be a few others, but that's not a terrific analogy is it?