Buildings rappidly increase in cost as the height increases. A 30 story building has vary close to the same amount of surface area as a 60 story building, but a 60 story building needs to support the full weight of the 30 storys on top of it, and at the same time have spare elivator capacity to reach the top 30 floors.
Yes, the cost is nonlinear with height, but my point is that the benefit to density can also be very high.
(I can believe that in the specific case of the Burj the cost outweighs the benefit, given that the skyscraper was built primarily to give Dubai bragging rights. But even if the infrastructure of a 124-story skyscraper is too expensive, that doesn’t refute the value of 60-to-80-story buildings.)
http://www.burjkhalifa.ae/language/en-us/the-tower.aspx