A huge factor will be where the project is being constructed. The local market for labor and materials is going to have just as much impact on this, and it will totally vary based on where the project is being constructed.
Scott is right, also the cost of land is a key driver in the cost effectiveness of high rise construction. 30 floors may be an economic stopping place in Houston or Tulsa where land is plentiful, but in Manhattan or San Francisco, economics dictate taller structures.
Interesting number, 30. I've just begun work on a 29-floor project. It seems here in Sweden, where highrise buildings are uncommon, there are *many* 30-floor buildings being planned right now. There's a word here "lagom" which means not too much, not too little. 30 might be lagom high for many places that aren't too crowded.