In 2005, Gary Barnett and Extell paid the Art Students League $23.1 million for 136,000 square feet of air rights to be used for what will eventually become the tallest residential tower in the hemisphere. But then last year, the developer decided that they needed a little bit more so the tower at 215 West 57th Street could cantilever. And that little bit more6,000 square feetcost a whopping $31.8 million. "They kind of got me back for the good deal I got years ago," said Barnett. [NYT; previously]