There's a lot that goes into assessing the value of a link. Knowing the value of a link is important, because you don't want low-quality links nor do you want to waste too much time, money or both on links that aren't truly worthwhile. The following are ten factors to take into account when assessing link acquisition targets.
You want links from places with significant authority and ideally trust as well. Not only domain-wide, but for the specific page in question as well. The more the merrier, but focus on the sites that have equal or higher authority than yours. Run a free Open Site Explorer report on the domain. For this example we are looking at SEOBook.com.
Alright, so not all links need to be relevant. Ideally most will have some form of relevance. Avoid getting many irrelevant links, as they don't really add that much value. Too many can be negative. The truly relevant ones are where it's at! Try to find the cream of the crop, the links from domains that have backlink profiles that include the leaders of your industry. For example, that would be like SEO Peng getting a link from SEOBook.com or SEOMoz.org.
Number of existing external links on page
The less external links on the page the better. This means more authority will be passed your way, the more links you're sharing with, the smaller your piece of the pie.
Relative difficulty for competitor to replicate
Can your competitors get links from the same source? If not, then this link is gold! Get as many of these as possible! If they can easily replicate the link, then you should still acquire it as the competition probably will in the future. Since your link campaign will probably start with you scouring through all your competitors' backlinks, this will probably be one of the first things you do.
Is the link going to be tagged with so it passes no authority? Don't waste too much effort going after these, but you'll need to get them too to keep your link diversity at a healthy ratio. Install SEO plugins to quickly point these out as you surf around. I'd recommend SEO for Chrome for, you guessed it Google Chrome and SEOBook's SEO Toolbarfor Mozilla Firefox. You can enable settings in these to automatically highlight links as you surf around.
Direct traffic potential
How many direct visitors is this link likely to send? Are these the types of visitors you want? How likely that any of them will result in a conversion? If it has no direct traffic potential, then it better provide some SEO value! Obviously there's no way for you to know how much traffic a site gets unless you have access to their analytics. For this reason, we'll have to rely on third party traffic estimation services like Alexa.com and Compete.com. You can use the SearchStatus Firefox Plugin to see these estimates for every site you browse, then you can hover over each for specifics. This plugin also allows you to show Google PageRank and MozRank if you're interested.
Location on web page
A link within the main content area is ideal. This will pass the most value to your site. Links from sidebars and footers are not considered as important. People are more likely to place paid links in these areas so they have been devalued relative to links placed in the main content area.
Anticipated link duration
How long is this link going to be around after you acquire it? Indefinite links are what you want. You don't want a temporary boost.
Relative control of link
How much control over the link do you have? Can you choose the anchor text and placement? Are you able to tweak the link in the future as it fits your needs? In most cases you will not have any control.
How likely is it you'll be able to even get a link in the first place? Is it really worth the effort to pursue? Money is valuable. Time is money. Keep the costs low unless the link really deserves it!