Green roofs can have hidden consequences. The structure of the roof has to be such that it will hold the wieght of the dirt, plants, etc. and subsequent moisture that will not be shed from the roof as quickly do to the retention of the planting areas. If the roof begins to fail you will have a tremendous expense associated with the repair. Give it some thought and be sure to have a structural analysis done of the structure before going forward.
Green roofs are a neat idea but come with some challenges. How do you install them and insure that your not adding weight the structure can't handle and also keeping it leak free. They are also subjective to the type of roof you have. You need an almost flat roof and are you willing to cover the whole roof to get the best savings The benefits of lowering attic temperatures don't always translate to savings. You still need to buy all the material and plants and keep the plants healthy. The savings in energy will probably be minimal unless you live in a place like Arizona where they are using the A/C most of the year. I feel that 50% savings is exaggerated for most areas and your return on investment would be many decades and the next buyer might not think it's a great feature.