making money from a directory is not really easy. You should have sound knowledge of SEO and a good pagerank too. but getting good pagerank is not enough because u have to maintain that pagerank so that people can trust ur site as a GOOD one.
also I think making a directory only for money is not good, you should also focus on adding QUALITY websites in ur directory because now a days everyone is making a directory and it is rarely seen that a directory has quality websites listed.
I have a little directory too , check it out
http://linksbucket.googlepages.com
I am not after money, just hoping to get some from google ads... people can add their sites for free
hope my points would be clear to you