Willis Tower, Chicago's tallest building, is about to break another record. When the world’s largest private equity real estate owner, Blackstone Group, pays $1.5 billion for the 110-story Windy City icon, it will shatter the $850 million record price tag for Chicago set in 2014 with the sale of a 60-story tower across the river.

Blackstone, the New York-based firm, has had its representatives visiting Willis Tower to perform due diligence, Crain’s Chicago Business reported on Friday.

The purchase is significant, even for a firm with $81 billion of real estate assets under its management, according to Crain’s report. Real Capital Analytics, a New York-based research firm, says it would be the 24th office deal in the U.S. to surpass the $1 billion mark, but only the fourth outside of the Big Apple. The 1,451-foot-tall property, formerly known as the Sears Tower, earns about $10,000 of income every hour.

At its projected sale price, Willis Tower is almost $400 per square foot for 3.8 million square feet of office space. For New York investors Joseph Chetrit and Joseph Moinian and the Illinois-based American Landmark Properties, who purchased the tower for $840 million in 2004, there is a hefty profit to be made on the agreement with Blackstone, Crain’s reports.

Willis Tower, which completed construction in 1973, doesn't command the rent prices of newer Chicago buildings. Only 84 percent of it is leased, and a new owner would need to spend money to attack new leasers to the empty spaces. Also, because the building is more than 40 years old, maintenance and modernization will require significant capital.

The observation deck on the 103rd floor provides a steady cash flow, in addition to retail and restaurant space, broadcast space and antennas, and building naming rights managed by Willis Group Holdings, a building tenant. Skydeck, the building's popular tourist attraction, generates more than $25 million in revenue annually, according to Crain’s report. Total building operating income for 2015 is estimated at $88.6 million -- that breaks down to $48.7 million for office, retail and restaurant space, $11.1 million from broadcasting, and the revenues from the Skydeck.