If you are asking "Is SQL Server free?", the short answer is , but it comes with a massive asterisk. Microsoft has done an excellent job of creating a "free" version, but it is designed specifically to hook you into their ecosystem without giving away the keys to the enterprise castle.
SQL Server Express: $0 (Production allowed, hardware limited) is sql server free
Students, hobbyists, and small desktop applications. If you are asking "Is SQL Server free
SQL Server Standard: Thousands of dollars depending on cores/users. the short answer is