It depends on how you host it. If you go in with a collective for a portion of a rack in a data center, generally the transit, while not "cheap" is unmetered and therefore the "per client" cost is nearly negligible. Obviously if your service becomes insanely popular you'll need to upgrade your circuit - depending on the provider and the data center, obviously. Most providers will gently coerce you into racking a higher bandwidth optics than you're planning to use, like recommending 10gbit gbics instead of 1 or 2.5 gbit.

However the price per megabit goes down significantly after the first 100, so, assuming you're merely sharing the cost with other rack users, again, it's negligible.