Knowing that you wont get any revenue on a directory from Adsense (because the majority of your traffic is webmasters), those types of directories are either: 1. Make profit from Featured or Sponsored Listings. 2. Market or sell a product related to directories or directory's niche. 3. Useless. Come on people, think, think, think. why so many questions?
I think at first you have to invest but once your directory achieve some good position in search engines etc then you get money by submissions and if your directory is free then you can sell it at good price if it's well ranked in search engines Saad