Assuming you're talking energy efficient, cost is 10-30% more, depending on where and what you're building. We had some here, ended up value equal to other non-green homes. However, you can comp regular homes, and adjust by the capitalized utility savings. The way things are going up, that might be very significant.
There is a "green" tract of homes now being offered here. Lots of publicity. Builder estimates cost increase is 40% but it included solar electric generating systems. No closings yet but many pre-sales, per builder. Prices start at $400,000.
you may also want to check out energy star rated homes down at the bottom of the page there is a list of builders (by state) who purport to have met the requirements to be able to build homes that meet EnergyStar qualifications.
These are not overall green homes, but have a 20-30% improvement in energy usage.
Point being if you can find any companies with building tract records you may be able to develop some longitudinal studies with respect to value!
(Hubby & I designed and built a couple Energystar rated homes in 1994-5... didn't make any money but learned a LOT about the process... and they are GREAT homes according to the current occupants - very happy campers compared to their neighbors energy costs)