If you have self-replicating machines, a megastructure may be easier to organise.

The great pyramids of Egypt may have been built by seasonal labour. Ramps were built to carry the building material to the top. Stone was quarried in standard sizes to build the structure, the internal rooms and the facing. Rubble was used to fill the larger voids. The craftsmen could work on finishing and decorating the chambers while work progressed on the outside. Once the process was started, they would know how many people and how many stones would be required for a day's building, and people could be sent to the quarries or the ramps or the pyramid worksite accordingly.

Suppose this was done using a system of self-replicating machines. They mine ore and produce metal to make more machines; they make more building materials or resources; or they work on the megastructure. The megastructure design might be a simple repeated set of arches, like Breughel's Tower of Babel. This would have enough over-design to survive subsidence, or faulty stones. You program the machines and let them get on with it. This is more expensive than building individual high-rise buildings, but the idea of 'expense' as we know it becomes meaningless if we have a small population and many machines.

There is an economic limit to building high buildings on Earth. A taller building takes longer to build. It is hard to sell lots in the building while it is still a building site. So the peak amount of money you have to borrow to get it built increases with height, even if you don't factor in the costs of building higher. If you have a wide, blunt megastructure, you could might live in one bit while building continued elsewhere. The ramps for lifting material might be hidden deep in the structure so you do not get the noise and the dust.

If you need another city elsewhere on the planet, what would you do? If the megastructure works, you would probably do the same again. It would then become 'the way things are done'.