Well...
#1. Positive image. Companies with overtly green buildings stand out dramatically and people will think better of that company because they made the extra effort. Its the same with all architecture. Great architecture = Positive reputation.
#2. More productive staff. People who are happy about going to work are more productive and work better as a team. Grumpy people who hate their concrete slab surroundings will resent their job and slack off.
#3. Energy savings. Green buildings aren't just designed to be friendly to plants. They're also made to save electricity by using less heat and AC. That means more saved money down the road.
And there is doubtlessly other reasons too. But the combined result outweighs the extra costs of green architecture. Its really more a matter of making the investment in the future as opposed to saving a few bucks today by building a cheap concrete slab of a structure.
