Question: just 1 click ..and it duplicates ALL content, settings, css, plugins..

im looking for a plugin that adds a new link to Sites admin; 

Edit | Dashboard | Deactivate | Archive | Spam | Delete | Visit | **DUPLICATE**... 

it should copy ALL settings/configs. 
and create a new subdomain instance in the **existing** Multisite

Answer: 
if you want a plugin that can have a default setup when you create a new sub-site then give this a shot [WPMUDEV New Blog Template](http://premium.wpmudev.org/project/new-blog-template) do not let the name fool you it does what your somewhat looking for minus the Duplicate button but that would be no longer necessary.
There are a number of plugins that backup the database as well as WordPress files.

Looking at the [plugins associated with the backup tag](http://wordpress.org/extend/plugins/tags/backup), the following look useful:

* BackupWordPress
* XCloner
* Backup and Move Plugin