If you are going to do a true side-by-side price comparison of a Zimbra rollout vs an Exchange rollout, you need to include the price of backups. Zimbra NE includes a great backup utility. Backup utilities for Exchange are licensed on a per mailbox basis. Zimbra wins here.
You also need to include the cost of server-side antivirus and antispam. Again, Zimbra includes this. I can't think of any Windows package that does not license these on a per-user basis. Zimbra wins here again.