Some directories have recently been removed from Google’s cache, while others
are not crawled very deeply. Additionally, some of them have not had their cache
dates updated in a great deal of time. Google might be trying to slow the growth
of directories by not counting the links from many of them. Make sure you check
the cache before paying for a listing.
Some of the directories will have a greater effect on relevancy in MSN or Yahoo!
than they do on Google, so even if a directory is not counted by Google, the link
price might still be cheap for its effects on other search relevancy algorithms.
Many directory owners are building multiple related directories. Some search
algorithms such as Google Hilltop are likely to automatically filter out some of the
relevancy score from a second directory if it is easily identifiable as being related to
the first directory.
The one-time listing fees make directories exceptionally appealing, but expect that
many directories will eventually be ignored by some of the search engines. In my
perspective, that cost is factored into the listing prices. I average out the link costs
for links from a number of sites. If I can spend $1,000 and get a dozen or two
dozen well-trusted links, then that is going to be a good buy for launching a site.
The reason there are so many different pieces of information associated with
directories is that for a good period of time, they were pure gold. They provided
cheap and easy marketing and a virtually unlimited ROI, but because they were
getting so abused Google had to buck the trend by coming up with ways to lower
that ROI.
Instead of just discounting some of the links, I believe Google may even place
negative weighting on links from sites they define as low quality. Since Google is
so much more complex and harder to manipulate than Yahoo! and MSN (and
Yahoo! and MSN still place great weight on directories, even on many of the bad
ones), it is hard to explain what directories count as quality links – it really depends
on your brand strategy, short term goals, and long term goals.

I think it is safe to say that when you look at directories such as Yahoo!, DMOZ,
Best of the Web, JoeAnt, and Gimpsy that they probably count as good links in all
the major search engines. Google still ranks many niche sites well, primarily based
on general directory links with few other citations.