The upgrade from OS X Server 3 (Mavericks) to OS X Server 4 (Yosemite) was a paid upgrade. OS X Server 4 to OS X Server 5 (El Capitan) was free. OS X Server is now renamed to macOS Server and is version 5.3 for Sierra. Based on that I would assume the upgrade will be free for High Sierra.
Perhaps you were using a really old version? The last three upgrades have been free for me. I purchased at the Mavericks release and I am now running Sierra. I am running the latest version and have never paid for an upgrade.