Upgrade Cost From Previous MacOS Version to Sierra
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