Talk about bad metrics, cost per link is one of the worst. Let’s talk about why, but first, I am going to outline how it comes up.
Typically what happens is that an SEO is performing work on behalf of a client, or the company they work for. They are diligently adding the links. Then the person managing the SEO does a calculation, looking at the money spent on the SEO, and the number of links acquired.
For example, if you have an in house SEO getting paid $8000 per month, and they obtained 40 links for you last month, your cost per link is $200. So why not try to drive this cost per link metric down?
The reason is that it will drive you to acquire crappier and crappier links. If you tell the SEO to get you links at a cost per link of $100, they are going to start looking for easier and easier ways to do it.
What the metric ignores is that all links are not created equal. In fact, a high-quality link can easily be 10,000, or even 100,000, times more valuable to your website then one of those crappy links. Looking at cost per link as a metric then could lead you to focus on getting 1,000 crappy links, instead of one really good one, which is a big mistake (we advocate a mix of quantity and quality by the way).
Let’s look at the major factors influencing a link’s value to your web site:
- Raw link juice of the linking page as crudely estimated by Google’s PageRank score
- Trust level of the linking domain, as very crudely estimated by Google’s PageRank score of the home page of that domain
- Relevance of the domain
- Relevance of the linking page
- Anchor text of the link itself
- Number of links on the linking page
One idea would be to try and create a more precise calculation of the link’s value. Since PageRank is a logarithmic scale, you can do the following math: Link Value = (10 ** PageRank) / Number of links on the page. Just to illustrate, if you have a page with PageRank 6, that has 123 outbound links, you get a score of 8130.081 for your link value.
This is a crude measurement, but it is certainly better than cost per link. However, this metric ignores other important factors such as the trust level of the linking domain, the relevance related factors, and the anchor text, and these are very, very important factors.
So if you are looking to measure the results of your link building campaign, stay away from cost per link. You can try the refined version that calculates link value as a metric, and that has some merit to it, but you still need to consider the other factors, based on a manual review of the links.
