This discussion is not asking if you use paid links. This discussion is not about whether paid links are ethical etc. It's just about what it says in the title, what Metrics should be used to judge the worth of a link?
Obviously, PageRank is a method many use, but I'm very skeptical of it. Having scanned websites for asking for free links, in certain niches I see the same sites plugging their anchor text with paid links. When I run their anchors on Google a lot of the time they are not ranking anywhere. My personal experience in obtaining free links from extremely low PR sites is that they do the job. Might take up to six months before they work, but they work.
At bottom, I think the same criteria for judging the worth of a paid link goes into the worth of a free link. What is it about a particular link that will make a site rank better? What are the criteria for judging the worth of a link?
Traffic as a metric
There are some webmasters who use PageRank as the basis for how much to charge. Some are asking from $700- $3,000 per year for a link off a PR 4 or 5 site. When you run their domain name through Quantcast and Google Trends for Websites it is very obvious their traffic levels are low. I know Quantcast is not accurate, but it gives an idea of traffic levels, especially when used with Google Trends.
So why is traffic level so important? I think it's important because it is an indicator, a metric, of how relevant the site is for certain keyword phrases. If relevance is the holy grail, then obtaining links from relevant sites is important. So I would suggest traffic levels as a metric for how important a site is if you are going to put a dollar sign on it.
Feel free to disagree. The above is only my opinion based on my observation. Your observations may be different and I would like to hear those. In fact, I will disagree with myself right now, I will offer a counterpoint. It is possible that what the site is relevant for might not be the big phrases, but if you look at the referral logs the core components of the keyword phrases are highly relevant to what is important to me, albeit in longtail form. So perhaps the traffic level doesn't tell the whole story? That maybe we should skip it and look at the keyword phrases used to find the site?
Or can the level of traffic still be used as a quality metric of how important (and effective) a link is?
Any other metrics to add to the list?