Which of these "costs" do not exist in suburbs? (except subway, the analog of which is highways to the suburbs). All of those are present in suburbs too.
Beside, you've already got your answer. Read the last sentence: This is a scalability issue. And my understanding is that it's difficult and expensive to build these things to scale to urban densities.
So your point being that to the service the same population it costs more if the population is concentrated in a smaller area like in a city, rather than dispersed in suburbs? I hope I am reading you correctly.
I'd submit that the experience with broadband/cell phone costs+coverage in the US vs. other countries shows that it's cheaper to provide services if population density is higher.
I believe you'll find that's also incorrect. The trouble there is that you can put only so much radio transmission power in a given "cell" on the network. In order to provide proper service to high density urban areas, you have to build more towers (more overlapping "cells") than in rural and suburban areas.
Now, you may end up having no higher a ratio of towers to subscribers than in the suburbs. But what you do have, is a lot higher costs to rent an easement for the tower. Renting antenna space from a building is a lot more expensive than an easement in some back yard.
Beside, you've already got your answer. Read the last sentence: This is a scalability issue. And my understanding is that it's difficult and expensive to build these things to scale to urban densities.
Density is the issue.