Yes, as long as you are hosting your primary domain with us, we can setup a subdomain for your account. The price to host the subdomain will depend on how you want to setup hosting.
If we will be hosting the content of the subdomain on our server, then the pricing and features would be based the hosting plan you choose. Since you will already own the primary domain, you would not need to pay a domain registration fee for the subdomain.
If the content of the subdomain will be hosted on an outside server, then you will need to request that we add an A record to your DNS entries. There is no charge for us to do this.
If you are hosted on Cloud Sites, another option would be to set up a sub-directory, ie. domain.com/directory/. You can setup unlimited sub-directories and there are no extra charges to do this.