The API is a paid service, like all the other APIs.
Costs are relatively fixed outside of infrastructure, and potential customers are any number up to and including the internet-connected population of the world.
The marginal cost of a new subscription is way less than they charge. The more they sell the less they lose, even if they're still losing overall to gain market-share.
That is what is the upgrade cost to expand capacity as new customers are added. If for example adding 1 million new users requires $200,000k in hardware expenditure and $20k in yearly power expenditure, but your first year return on those customers is only going to be $50k, you're in a massive money losing endeavor.
The point here is we really don't know the running and upkeep costs of these models at this point.