I've also run a relatively large (Chicago-scale) ISP. Here, we worked from minimal budgets and headcounts based on municipal networks in other states with published budgets; the costs are not minor. What I think I did there was misuse the term "fixed cost"; I was referring to the expenses we incur no matter how many residents opt in to the municipal network.

Note that the economics here don't appear to work even given that we already have a municipal fiber network covering the whole municipality.