But a much, much cheaper and much easier and simpler to use software is TripMode - https://www.tripmode.ch/ ...
While it is not marketed as an outbound firewall, it does a great job as one. It isn't as sophisticated as Little Snitch and doesn't offer fine-grained levels of filtering but that's a plus for those who are not advanced users - it simply allows or blocks an "app" from connecting to the internet. It can also monitor your bandwidth usage.
(The Mac version is very stable, but I found the Windows 10 version to be a bit buggy).