Domain names need to be registered , what usually means that certain registration fee has to be paid and then renewed each year.
Network Solution (using also the name InternNIC) originally charged $100 for the first two years of .com, .net or .org names, later it was $70 for two years, i.e. $35 per year. After the monopoly ended, prices dropped. Now if you pay $15 or more per year, it is expensive.
The are many registrars when you can register a domain, we will feature several of them here.