Galaxy usage is priced by the Container/Hour. In the most basic example, if you host one app in one 512MB container in the US-East AWS Region, you will be billed $0.04 per hour that container is running (measured to the second). Based on the length of each month, this amounts to ~$29. Billing begins when you deploy or increase the number of containers. If you want to stop billing, stop your app, and billing will stop even though the app is still there ready to be started again.