I haven't looked into it yet so I don't know about cost, can't imagine it'd be too bad considering the size of the roof.
I quick google of how to build one, this was the first hit http://www.ehow.com/how_6545147_make-living-roof.html
I quick google of how to build one, this was the first hit http://www.ehow.com/how_6545147_make-living-roof.html