It is all about the traffic and what that traffic typically does on your site. If you can show that you draw a lot of people to your site, have a high ranking on Google, and that those visitors can easily be converted into banner clicking and eventually sales, your site becomes more expensive.
If you can draw a crowd of 1 million visitors a day but 2 of those users click the ads, it won't fetch as much as those who can draw 10 million visitors a day and can convert 85% of those visitors into clicking-throughs and eventual buyers for an advertiser.
Good search engine optimization/affiliates linking to you >> good google placement >> tons of visitors >> convert a high percentage of visitors into buyers >> more expensive your site can be worth.
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