I came across a green roof just yesterday. It had a leak around a vent that was no longer in use so i ordered two bundles of empire green blend shinlges and some plywood and took out the vent. lol
Got one planned for the spring. Already have the base on the roof. TPO single ply. Using the LiveRoof system.
Rough estimate at $25 per sq. ft.
http://www.greengridroofs.com/system/gsa.htm
Never done one, but you could call a landscaper! Just a thought.