Anything you want it to be. That's why it says "Sub-domains okay!" next to all of the options.
Since all plans come with unlimited domains and sub-domains, neither takes away from anything.
If by $9.95, you mean the registration, then that only applies to domain names. You can make as many sub-domains on a domain as you want.
Save $96 at Dreamhost with the 96DOLLARSOFF promo code.