The biggest cost of such an effort is navigating local bureaucracy. You will hit a different permitting and approval process every few miles. Every neighborhood council has an opinion on how you should dig the hole. Every town wants to do an environmental study, economic impact study, town halls and endless deliberation. There are always vested interests in play. You can use only a specific kind of union labor. And then a month into deployment some idiot fixing a pipe will cut your cable.

Despite a massive investment Google itself couldn't pull it off with Fiber, and I doubt startups have much of a chance.