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All blogs show their Alexa ranking which is calculated by how many unique visitors per day the blog receives combined with how many page views it gets. For example, an Alexa ranking of #1 means it's the most popular site on the Internet where an Alexa ranking of #1,000,000 might mean the blog gets about 200 unique visitors per day with about 1,000 page views.

Therefore, by looking at the Alexa ranking, you can get a really good idea as to how much traffic you'll get. The price per article will be a function of a blog's Alexa ranking.