I have a Git GUI desktop app that I want to integrate with various services, including Bitbucket Server, using the REST APIs. Is there a way I can develop for the Bitbucket Server API without having to buy a license? I could try the 30-day trial, but I’m sure it would take me longer than that to get it working 
You must buy at least a $10 license if you want to obtain the source code.
However, you are free to develop on the SDK free of charge. The SDK’s commands ‘atlas-run’, ‘atlas-run-standalone’, and ‘atlas-debug’ all are started with a 1-day developer license. This seems to be instantiated each time you start the application with the SDK commands. This is suitable for developing ad infinitum.