Beyond any anticipated direct traffic,
Unless the directory is relatively decent AND niche, I wouldn't expect any traffic. here's a couple of data points from my perspective:
- I have a niche directory where I provide free listings. I've had listings call me and thank me for sending them clients. So it drives some small level of traffic. But probably not a lot.
- I've had clients get listed in places like Yahoo! and similiar. They either drive no traffic, or a level that's so low it's certainly not worth the cost just on traffic alone.
- I see Yahoo and similiar as strictly for use for SEO purposes. Personally I've not paid the $300 for Yahoo, I've generally got something else I think is better money for the $300. If I was doing it for something else, I might recommend it as an entry level link. For me, I believe I can get better links for free.
And I have a friend who has a directory in my niche as well, and I know his directory also drives some small amount of traffic.
So my experience, if you're looking for traffic, I'd keep the money in your pocket and go looking for high quality niche directories long before I'd look at a general directory.