Nanobox is a portable "micro" PaaS, we don't provide servers, we integrate with your host of choice and provision servers for you. The cost and number of servers your application needs/uses is between you and your host (AWS, DO, etc).
The $99 you pay to Nanobox is for the use of the Nanobox Dashboard to manage your application on your host (Nanobox Desktop is free).
Nanobox is free for developers to use for personal and open source projects.