To first save money on domain names you need to decide whether your site needs a well known Top Level Domain, or just Sub-domain.

Example of a Sub-domain where you would not require a TLD and would save money. So Say you own a football club and already have a site on football, for ease lets call this football.com. To save money you could create a sub-domain, like myclub.football.com. This would be free if your web host or domain provider allows it, most web hosts do.

Without a doubt this is the one of the easiest and cheapest (well free) ways to save money on hosting extra domains.

If you need to register Top Level Domain and still want to save money, then you will find that not all domain extensions are as pricey as others. Although they may not be as well known as most common domains.

You may find that the most common names will be ranked higher in search engines. E.g if you were polish and create a site written in polish language, having a .pl extension on your domain maybe rank you higher in some search engines for polish search terms. If you look at the excel spreadsheet you will see that some are cheaper than others.