Text Link Ads (TLA) has a tool for estimating the value of advertising links over their system. Skype, for example, could earn $11,880 from a banner add at the top of their skype.com page or $10,764 from a side bar link (categorised under Other -- the link price increases if categorised under technology). Sell more than one link and the unit price declines but potential total earnings increase. 10 sidebar links, under the Other category, potentially earn Skype $4,036 per link or, in total, $40,360 per month for all links.
Google,Yahoo etc return messages such as "priceless", "too many zeros", "more than you can afford" and so forth while msn.com returns guess who? Gambling sites are paid the largest amounts for selling space. A popular food blog like Chocolate and Zucchini is $51 per link (side bar, single page) for 10 links or $510 per month in total. Alas this is not for all bloggers as the software doesn't work with Typepad blogs or Blogger (without a dedicated hosting account where you publish your Blogger blog) -- though you still estimate value of links with the tool. But it will work with some blogs. Then again the links have to be sold for revenue to accrue.
Publishers can also choose to use TLA's automated publishing program to sell ad space. It operates on a 50% revenue share.