The sale of the Chicago office tower at 300 North LaSalle St. for a record price of $655 million has left a number of real-estate professionals rubbing their eyes.

Like most cities, Chicago is suffering from a weak office-leasing market. The city's vacancy rate at the end of the second quarter was 18.5%, up from 17.4% during the same period last year, and effective rents have been on a downward spiral for more than a year, according to Reis Inc.

But last week, KBS, a Newport Beach, Calif., real-estate company, purchased the 1.3 million square foot office building overlooking the Chicago River through an unlisted real-estate investment trust for about $500 a square foot. That is the most ever paid for a Chicago office building on a square-foot basis, according to Real Capital Analytics. By comparison, the Willis Tower, the tallest building in the U.S., sold for about $840 million in 2004, or $244 a square foot.

So what gives?

The answer is that in the current global economy, there is a widening valuation gap in commercial real estate between office buildings with a lot of vacancy and those that are close to fully leased with financially strong tenants. Many landlords with high vacancy rates are watching the value of their buildings fall with declining rent and the growing difficulty in filing space.