A subdomain costs nothing. It is created through your cp andis accessible as
http://www.subdomain.yourdomain.com
In example, if you need soho to build a store, create a subdomain called "store" and instal Soho in this subdomain.
If you need a domain that can be independently accessible, as
http://www.newdomain.com then you need to purchase (register) this new domain name, usually $19.95/year, and add it as addon in the same account, then install Soho in this new domain.