At the moment the system is in public beta and is free. After that there will be a free tier for small projects with at least 3K views a month. Above that the service will be for a price of approximately 1$ per 10K visits a month. All early adopters will receive discounts. The details are in the works.
It is still much cheaper than running it on a server, which alone will cost at least 3-5 dollars a month, let alone time spent for a basic server which costs, for instance,
There are three reasons to talk about it upfront:
There is no such thing as a free lunch. If you are not paying, you are the product, not the client.
Running the servers require costs proportional to its use and, therefore, the good it provides to the clients (you).
Maintaining and developing the project requires time and resources. In order for the project to live long (which is especially important for infrastructure), it must be continually maintained and developed, which in turn requires resources.