As a rough estimate, an always-on, 1 megabit-per-second tap into the internet backbone in downtown Atlanta, bought wholesale, costs an ISP $10 to $20 a month, according to the research firm TeleGeography.
http://www.wired.com/news/wireservice/0,70895-0.html?tw=wn_index_6BellSouth's chief architect, Henry Kafka, uses the assumption that the cost of providing a month's worth of data to the average user, about 2 gigabytes, costs the company $1.