Investment firm Blackstone Group plans to buy Chicago's Willis Tower for $1.3 billion—the highest price ever paid for a U.S. office tower outside of New York. Blackstone reached the deal for the building formerly named the Sears Tower designed by local architecture firm Skidmore, Owings & Merrill over the weekend. Blackstone executives say they plan to invest in the building's retail and upgrade its glass observation deck to make the office building more of a tourist attraction.

This is a huge profit for current owners, New York investors Joseph Chetrit and Joseph Moinian and Skokie, Ill.–based American Landmark Properties. They paid $841 million for the skyscraper in 2004. But really, the $1.3-billion deal for the skyscraper isn't sky-high. The price per square foot is about $340, a small amount compared to what trophy towers fetch in other major cities. Read More