That depends on what exactly you need to add. It could be free -- there might be a charge.
Do you need domain Registration or would you like to transfer Registrars? Is the site merely a DNS pointer to your main site or does it serve fully independent content?
If you want to add another domain that will serve the identical web page as some other domain you already have, you don't need to add a whole new site. You can add extra domains like yourname.net and yourname.org to your main yourname.com site for no extra charge other than the registration fee. What you want in this case is a web site alias. These are free for ProWeb customers. Please check the JTAN http://www.jtan.com/services/pricing.php3 document for the latest pricing on these features and packages.