Upgrade Cost From Previous MacOS Version to Sierra
Solution 1:
Apple last charged for upgrades at 10.8 Mountain Lion. Major upgrades have been free of charge since then. The system requirements are publicly listed.
- https://www.apple.com/macos/how-to-upgrade/
Mac computers introduced in late 2009 or later can be upgraded to macOS Sierra.
You can also rely on Apple providing you a clean copy of the OS that shipped with the hardware via support download if it's Lion or later.
- https://support.apple.com/en-us/HT204319