Nexus is free-of-charge and licensed under the Apache 2.0 license. It is however not publically available as the repository is private and the NuGet packages are hosted on a private feed.
It may seem counter-intuitive to have an open source licensed code base in a private repository but the reason for it is that we want to know who is using it and be able to help out. It's possible that we at some point will release Nexus completely public. If you want access just hit us an email at [email protected]!
The important thing for you to know is that once you've gotten access to the repository and NuGet packages your access to it is protected by the Apache 2.0 license. Which means that you will forever have the right to the code and do whatever you want with it as long as you follow the license terms.