Good luck with the project ... a hand-edited directory is a big task indeed! You can make money by running AdSense or other contextual based advertising on the page.

Something you might consider is within the form to "Add a Link" to the directory, you might have an added option: "If link is accepted, make link bold for $250" or something like that.

Another option is next to every link in the directory there could be an icon for: "Do you own this link? If so, feature it in bold at the top of our category for $50 / week"

You could then get subscribers that way The benefit would be that you're only allowing advertisers that you would have normally accepted within that category anyway.