Building a fiber network is expensive - much more expensive than running it for couple years. The first two municipal fiber projects I looked up cost the taxpayers an average of $3,200 per household to build. Whether you want it or not, every resident had to pay to build it and that's the bulk of the cost.
The monthly subscription fees which cover the cost of maintaining it after it's built are a small portion of the cost. Rather than listing a muni such as Lake County as $40/month, if the "study" were intellectually honest they'd list it as "$3,200 up front, plus $40/month". That's the actual cost to residents.
Promoters of these schemes hope that one day subscription revenue might pay back the cost of building the network, but that's never happened yet, to my knowledge.
For *some* projects, bonds are used in such a way that taxpayers will only have to pay the shortfall, the difference between what subscription fees bring in, minus expenses vs the cost to build it and financing costs for that cost. They hope that shortfall might be zero, but often it's thousands of dollars per household.
Most often it's a mixture of bonds, where taxpayers pay the shortfall, and direct tax dollars. For example Lake County was promoted as "financed by bonds, won't cost the taxpayer a dime", but in fact they've spent $15 million in local tax money $1,400 for every man, woman and child in the county, whether they get the service or not, plus state and federal tax dollars.
Chattanooga is probably the biggest "success" hyped by muni fans, and with good reason - it's not losing millions of dollars a year like some are. In fact, it's just started to make payments toward maybe eventually paying back some of the $97 million of taxes used to build the network. That's the big success they point to - so far taxpayers are only out $90 million and it's not getting worse at the moment.