The zBox is offered as part of a complete online backup service known as ZettaBits. The zBox runs on your local network and automatically backs up data to Amazon S3. Once you commit (I'm not sure if the proper term is buy, lease, or rent) a zBox and place it on your network, data stored locally will end up safe and secure in S3.
The ZettaBits service is offered at standard and pro levels. The standard zBoxes feature 30 to 690 GB of local storage and can handle up to 5 simultaneous users, at prices ranging from $49 to $299 per month. The pro zBoxes feature 140 to 690 GB of local RAID-1 storage, and can handle up to 100 simultaneous users, at prices ranging from $199 to $499 per month. There's more info on the Service Plans page.