Thing's I've learned:
In complex buildings, payback period for it to be sustainable are normally around 4-8 years. The extra cost can be as high as 50% if the building itself.
The most important thing to consider in complex building is the air conditioning. A good selection of air conditioning system will reduce building energy require as low as half as it was.
The second best argument for a business case for a sustainable building is the worker's productivity. Increment of comfort, may it be thermal, visual,...., will increase productivity. If an office has 100 employees, and these 100 employees are paid AUD60k/yr, an increment of 5% in productivity, will generate an extra AUD300k/yr for the company. Given this idiosyncratic calculation, you'll get the payback of your whole building in 12yrs.<--- this does not include air condition or 'sick leaves'.