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For a general reference, A/E/C firms will advise clients 1-3% of the budget should be dedicated to "green" activities (more than the design contingency!) Not too sure how that translates to home builders (taking into account retail pricing and a lack of access to particular amenities larger contractors have access to). First-costs almost always come with sticker shock, but tend to recover their costs in a few years.

Home builders should be firm in their decision to build a sustainable project and stick with it. You will see yourself cutting and slashing parts of the project due to budgetary constraints and sustainable items are usually the first to go.

For homeowners. the costs do ultimately balance out over the long term, but a lot of green features take between 15 and 25 years to pay for themselves through lower energy costs (and that is with government subsidies), far longer than the average tenure in a house (about 7 years).

Thankfully, commercial and educational buildings are built for the long term, so investing in green construction does make sense, and the increased use in green construction in such building will ultimately have a trickle down effect, lowering the costs and shortening the cost recovery period from domestic construction.

Another significant barrier in residential green construction, however are local zoning ordinances -- a lot of the more cost effective green technologies are currently not allowed in many communities. With a greater attention being placed on the importance of green construction,m this is, thankfully, changing, albeit slowly.

Wouldn't the relative cost also depend on how you define "conventional" home and what you call "green"?

Take something simple like good insulation and double-glazed windows, both of which do a lot to reduce energy use. Those used to be rare; they're now pretty standard in tract home construction. Ceiling fans help with lowering AC and heating costs; again, standard or cheap to add. Choosing a house oriented in a climate-friendly way (south-facing in the north, but not around here) costs you nothing extra.

If you want bamboo floors, that'll cost you in relation to basic builder-grade flooring in a middle-class tract home, but it could come out par if your comparison is fancy wood in a one-off luxury home.

If the issue is the sustainability of the wood used to frame the house... most wood in the U.S. is actually farmed and renewed, so this may be a false issue. But when you want something other than the builder's normal supplies, that'll cost you. On the other hand, if you're building your own house and scavenging for reusable lumber, you could come out ahead on cost. (And if you want to use mud walls, the building inspector will just say no, so your costs will include promoting a local initiative to change the building code. Ouch!)

Solar power would be a special item in most places (so more up-front costs), but in the right climate, it could pay for itself pretty quickly. (Why don't builders make it standard here in AZ? We have the sun.)

That's a tough question - some building tactics are just common sense, and don't cost that much more, like using deep overhangs to shade the windows, and good insulation. We found plenty of energy-efficient appliances that will save us money in the long run. But if you're going for extra credit, and trying to totally avoid fossil fuels, that will definitely add up. We're going all electric, and using geothermal heat for our new house, and that system will probably cost $20,000 more over a propane forced-air heater. I don't think we'll ever recoup those costs, but if the costs of solar panels come down someday, we'll have zero energy bills!