"The lower cost is probably due to the fact that muni systems don't need to make obscene profits to "return shareholder value" and payout huge C-suite bonuses?"
You can tell yourself that, but you'd be wrong. Those are relatively small beer, compared to the running costs, financing costs and depreciation of any infrastructure network. The key driver of cost in either public or private sector is (a) whether you're competent at designing, building, financing and operating such networks, and (b) whether you have good asset utilisation.
If we differentiate between "excess shareholder returns" and the cost of capital that even a municipality should be accounting on commercial assets, then as a general rule the greater efficiency and focus of a commercial outfit will outweigh the director's and Wall Street's incremental greed. If cities want a finger in the pie, they'd mostly be better off forming a corporation to own the assets (giving them a lot of control, and possibly access to cheap municipal financing), and then contracting out all design, build and operation.