> I'm not sure how costs of sewage etc. change with increased population density. Pipes have to get wider per person, but low-density also means they're longer.

Probably accurate to say that in general, the great majority of the cost of infrastructure is in labor, not materials. It doesn't matter if pipes are a little wider or longer; the labor of digging up dirt and installing pipes, and doing that over time as things need maintenance, would far exceed.