I don't think it's worth the money you'll spent od AdWords. At least in a case of web directory. You'll pay let's say $0.10 for a visitor and what you get? Maybe $0.05 from click on adwords, or, more probably the visitor will just leave to one of pages in your directory.
I think it is worth it only if you have a CPA offer connecting where the person who while before leaving your site fills in a small survey with their name and email address which is a lead for the CPA offer. You can make money that way. I hope this helps. You require a strategy in place before you start adwords otherwise it's a waste of money. Rep me with a +1 if you like my idea. Thanks.
I use AdWords to step up traffic but you should be ready to spend a minimum of $1,000 per week. Knowing how to effectively use and manage AdWords is essential or the costs can skyrocket. Don’t expect I good return on your investment. I would only recommend it to increase traffic for your submitter’s websites.
Maybe if you bid on good adwords like "submit to paid directory" and have your visitors pay for submitions, than it could work. You should then keep close track of your conversions and stop or reconfigure before you spend to much. I do think directories are getting a little old. They are only usefull, because so many people still believe in them and owning one can create quite a lot of backlinks to the directory OR to another site you own (which may be a pretty smart way to do things.)....
The best place is directory rating websites. Advertise there. You can use AdWords to show up your website on other directory rating websites. This might help!