To add a little more to try to alleviate your confusion. There are fee's for 2 basic things: HOSTING, and domain REGISTRATIONS.
As DH_Elle has repeated there is no additional HOSTING charge as long as you have a hosting account. You can add as many domains/sub-domains to your hosting account as you have control of. You can use domains that were registered anywhere so long as you can set the nameservers to dreamhost (or in some cases make manual DNS entries for the domain).
If you have to REGISTER another domain, you do have to pay for that, but it doesn't have to be a dreamhost registered domain, you can register additional domains with any registrar.