**What should i be looking for to decide if these directories are worth paying for a link? **
Be very careful to ensure the directory does not violate Google's Guidelines. In my experience, most paid directory listings have no value whatsoever, and may actually harm your site. A few examples of requirements for a paid directory link:
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the link fee must be for an evaluation of the site, not for the link itself. In other words, any site which offers any form of guarantee to be included in the directory is violating search engine guidelines. It is highly likely Bing and Google have devalued all links from the site. This can be confusing because the site may retain its own PR, but the outgoing links are devalued.
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the link fee must be non-refundable. If the fee is "guaranteed inclusion or your money back" then the directory violates guidelines.
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the site must offer solid selection criteria. The idea is the directory separates "good" sites from "bad" sites. One example would be the Better Business Bureau directory which sets standards for membership.
There are numerous articles which cover this topic in detail. A helpful link from Matt Cutts: http://www.youtube.com/watch?feature=player_embedded&v=rKUlVquEImc
what about page rank of the page that it's on? Does that matter?
Sure it matters. Understand when you refer to PR you can only see "toolbar PR" which is often months old. There are directories which manipulate a page's PR, then as soon as Google updates the toolbar PR the directory makes changes and the PR is then worthless, but they will earn revenue for months based on the falsely assigned PR value.
For this questions and your others questions, there is an excellent detailed discussion you may find helpful: http://www.seomoz.org/blog/understanding-link-based-spam-analysis-techniques
** i've heard that one page rank 5 link carries the weight of hundreds of page rank zero links. Is this true?**
PR is assigned on an algorithmic scale. Yes, higher PR is worth significantly more then a similar page with lower PR value. There are many factors which affect a links value including the position on a page (header vs footer vs content), whether the link is "above the fold", uses anchor text, etc.
What does it mean if the page rank is unranked? Is it worth paying for a link if the page is old and is unranked?
The term "unranked" may have various meanings depending on the tool you are using. It could be a new page which has never earned a rank, or it could be a penalized page. If you earn a link on a quality new page, then it may already have earned a decent PR ranking but it is only visible to Google.
I read in an SEO forum that there are many other things i should look at in a directory besides page rank. Obviously traffic is one but what about other not so obvious things?
The most important consideration is whether the directory is useful. Will anyone actually go to this directory, perform a search and then find your site as a result. The second most important consideration is whether the directory is 100% legitimate. In other words, do the links offered pass value. You may disagree with the order of these two criteria, but either way they should be your focus.
How many directory links at one time is too many? how many is too many over a long period of time?
This is a judgement call and opinions will vary. I find there are very few directories of a general nature which offer any value. My preference is to add one general directory link per month until the quality directories are used. The total directory links to your site should be a very small (less then 10%) of links to your site in my opinion. Many SEOs use a higher percentage, a practice with which I disagree.