They only want money for providing the build. The source code and build scripts are free/libre.

From the website:

  How do I install Conversations?

  Conversations is entirely open source and licensed under GPLv3. So if you are
  a software developer, you can check out the sources from GitHub and use gradle
  to build your APK file.

  The more convenient way — which not only gives you automatic updates but also
  supports the further development of Conversations — is to buy the App in the
  Google Play Store.

  Buying the App from the Play Store will also give you access to our beta test.