I rolled all of that up into the cost of customer acquisition. The operating costs (bandwidth, hosting, customer service, fraud insurance) are tiny when spread across the entire customer base.

I guess it would be more accurate to refer to the cost of producing one more month of service rather than one more customer. Even then there are costs, but they're tiny compared to the costs of providing a physical service.

Contrast the costs of running Netflix now vs Netflix when it was distributing DVD's in the post (or the costs that Blockbuster had). Adding one more customer to Netflix costs so little now that it's effectively zero, compared to the direct costs of sending out DVD's in the old days.