Questions regarding developer program

Hi,

We are planning to build an integration with MyOB via business API. We are exploring the developer program and raise with the following questions. Kindly clarify.

1) Developer API access means during the development and Sandbox access we need to pay $ 110/month, develop, test and goes into production and are we done with paying $110/month?

2) Once our integration is completed, do we still need to pay even after customer started accessing the data via our integration? 

3) After development of integration, if we don't have a customer to use the integration i.e no usage of API and even then, we need to pay the developer program fee?

Thanks in advance.