I keep seeing this argument and it honestly just sounds rather lazy when a few seconds of Googling will point you towards build services you can use without having to purchase a Macbook (for instance, TravisCI has an OS X build environment). Or if you're opposed to cloud-hosted build tools for some reason you could ask users/the user requesting a macOS binary would mind working with you to test your build script(s) for compatibility and mention that Mac users will have to build from source in your README. This whole "well I would if Macs weren't so expensive" sounds like "I just don't want to support macOS" because you have options if you actually were interested.