Zimbra sells annual and perpetual licenses for the Zimbra Collaboration Suite (ZCS), and second-level support for systems administrators managing ZCS installations.  With different features, there are different editions of ZCS, ranging from free (the entirely open source version), to the Network Professional Edition and its options such as Zimbra Mobile (Windows Mobile and Palm), BES Connector, and Zimbra Archiving and Discovery (HIPPA/SOX compliance).   Note that 01.com only offers the Network Professional Edition in its shared/cloud hosted environment, with a free, 30 day no-obligation-trial and automated sign-up.

Zimbra charges a standard rates for the Network Professional Edition. If you allow your paid, commercial Zimbra license to expire, it reverts to the open-source edition.   Note that this is the licensing cost only, it does not include, for example, hosting, hardware, operating system, systems administration, and help desk support, it's only the license to run the software.

Zimbra sells licenses directly, and through a network of Zimbra value-added-resellers (VARs) such as 01.com, who is also a hosting provider, discussed below.   Zimbra ensures that pricing is the same through either channel, direct or through its resellers.  The VARs typically provide the services to help organizations deploy Zimbra themselves, services such as dedicated hosting, hardware specification, planning, upgrades, operating system management, migration, integration, off-site backup, advanced spam and virus (supplementing the built-in anti-virus and spam), programming, integration, help desk services and a variety of best-practices training. 

Zimbra also has relationships with various hosting providers such as 01.com, who deliver Zimbra as Software as a Service (SaaS), a turn-key solution for organizations to rent the use of Zimbra on a per mailbox, per month or annual basis, securely across the internet.  The host with the most Network Professional Edition mailboxes, 01.com delivers Zimbra SaaS to organizations in more than 70 countries from two hosting centers in Europe and the United States.  See some differences between 01.com and other providers here.