$2.5 billion and 2 years building is what you need to grow a "Vertical Forest" (trans. of "Bosco Verticale"), a public-private investment expected to become one of the most important elements within the redevelopment of the Porta Nuova district in Milan (Italy).
Known to be Italy's most polluted city, Milan wants to get a new face through this Green Building project. The 260 feet and 367 feet high towers include 800 trees between 9 and 30 feet tall, and 15,000 plants, such as vines and perennials, which have been used to cover the ground. The buildings come with a water recycling and irrigation system as well as photo-voltaic solar cells. (source)