There is no charge to move a domain to another iDotz user. However, before you do so, please make certain that you update the contacts and nameservers.
To move a domain, log into your account, click on the domain name. On the subsequent page, look for and click Push Domain to access option. You will need the username and contact names for the new user to successfully push the domain through.
NOTE: Certain registries may not allow for a change of contact or may charge for the update. Refer to individual registry for their rules and restrictions.