Two values that you should quantify are referral traffic and link juice. Referral traffic is self explanatory. Link juice has many factors to consider such as the 'inbound links' to the page, the cached date (how fast the page is crawled & indexed), relevancy and authority. The best way to determine the amount of link juice flowing to your page is to track any increase in rankings on the anchor text.
PR is also a measure that you can use as well. But the toolbar isn't very accurate and should always be taken with a grain of salt.
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