Carlana, truthfully, the least expensive (yet still decent) company for registration that I am aware of is
GoDaddy. It can be as little as $7 per year to register a domain through them and they give you complete access to your nameservers, etc. (which many of these cheap registration companies do not do and it is so important as a business owner to have immediate and easy access and control over these - in case she ever wants to move her site, etc.)
Just my 2cents