The WSJ has an article about the reality of costs for going green in the home.

In fact, earning back the green premium can take years, not counting rebates and incentives that may be available from government agencies. Enermodal calculates a payback period of more than 10 years for the most extreme green measures, including super-efficient furnaces and water-thrifty faucets. Systems integrator Solar Depot estimates a solar hot-water system will pay for itself in eight to 10 years, depending on the climate, site and home size; a solar radiant floor-heating system will take five to six years. But considering the average U.S. homeowner lives in a house only seven years before moving, many will need more than the hope of lower utility bills to inspire a green remodeling. (And some green products, such as bamboo floors, don't save any money.)

Data centers going green shouldn't run into the same problem, as a 10 mW data center is the equivalent of 10,000 homes, so energy ROIs are much quicker.