Cheapest (Even Free!) Domain Name Extensions
Common domain name extensions such as .com, .net, .org can be bought for less than ten dollars. But there are other, less common domain name extensions that cost a fraction of this price. Some domain extensions can be bought for a dollar or less and some are even free.
Free Domain Names
Want a free domain name? get a .tk domain. Domain names ending in .tk are free. As with most free things, there's a catch. Actually there's a few catches. One is that free domain names are used by spammers, phishers, cybercriminals, etc. This means that a domain ending in .tk is automatically going to be blocked by a whole lot of email servers and web filters. They are untrusted.
Another catch is that the .tk registrar is not exactly reputable. They will take your domain away from you if it becomes popular. That way they can make money from it themselves.
So a free .tk domain might be a good idea for a little hobby website that you are developing, but certainly should never be used for anything that you want to keep forever.
Cheapest Domain Extensions
Beyond the somewhat sketchy but free .tk domains there are a number of cheap domain extensions that you can purchase from more reputable registrars.
The above are about as cheap as you will get when it comes to domain names. And if you plan to use the domain name for anything other than a personal site then you should definetly spend the dollar or two to register with NameCheap or NameSilo rather than use the .tk free domain which might get taken down at any time.