btw. how do you calculate the price for a listing? is the submission fee higher on "everything from A-Z" directorys or higher on directorys about a special topic?
Depending on what niche, perhaps. If it's a case of being too niche, you may have difficulty generating traffic.
The more links the better. I would think you would want as broad a niche as you have time to spend. If you can manually approve 5,000 links a day more general but if you have less time a more narrow niche with less categories.