That article only states that having individual office use three times as much space as open plan offices.

It offers no insight in studies how much a single office improves productivity and how that affects overall revenue of the business. I'm convinced it's more then three times of the lease for office space.

And keep in mind that open-plan offices are turning away candidates (see comments in this thread). They are losing talent for optimizing for real estate usage. This needs to be factored in as well.