I'll give you my view of Google advertisements on a web directory. Since more traffic increases the possibility of clicks, you can figure out that part by yourself.
Free web directories have more traffic than paid web directories do, and you may receive about 200-300 unique visits per day if you have a established free web directory.
30x200=6000 UV's a month assuming you receive 200 uniques per day.
30x300= 9000 UV's a month assuming you receive 300 uniques per day.
This as you may be aware of can bring a quite good amount on Adsense, I've had good results with low traffic web directories, I placed the G ads just below the navigation bar and made it 'less-disturbing'.
Optimize your web directory for SE's and try to target high traffic keywords that may expose your web site and bring you a good amount of traffic.
Meti
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