As Rayiner commented elsewhere in the thread [1] Boulder has a population of 40,000 households, which is the appropriate metric to use when people within a household regularly share a connection. It gets more expensive when you consider non-subscribing households, subsidized households, and operating costs. Payback periods quickly climb to 10-20 years or more, depending on how you do your math.
However, I think it should also be considered that a municipal broadband network adds value to a city in much the same way that a road network, electric grid, natural gas, or water/sewer network adds value. For a point of contrast, the city contains ~300 miles of roads at ~$5 million per mile, or a road network value of $1.5 billion dollars. That's about $40,000 per household! Other utilities have huge costs as well.
I believe that Internet access is becoming as important as transportation, water, and electric service for the economic health of a city.