Bandwidth transit prices, peering, and other data for for ISPs and the like tend to be highly classified (lol), but it's very close to $0. Take Steam for instance. They are responsible for a significant chunk of all internet traffic and transfer data in the exabytes. Recently their revenue/profit data was leaked from a court filing and their total annual costs, including labor/infrastructure/assets/etc, was something like $800 million. [1]

Enabling on site money transfers (as YouTube does) and taking a small cut from each transfer (far less than YouTube's lol level 30% cut) would probably be getting close to enough to cover your costs, especially if you made it a more ingrained/gamey aspect of the system - e.g. give big tippers some sort of swag in comments or whatever, stuff like that. It's not going to be enough to buy too many [more] islands for Sergey and Larry, but such is the price we must all pay.

[1] - https://www.pcgamer.com/gaming-industry/valves-reported-prof...