Businesses frequently make decisions based on cost-benefit analysis: Will a project save or earn more money than it'll cost to implement? There's no reason you shouldn't use the same type of analysis for your home project.
Ask yourself how much green building will save you in energy and/or other utility costs. Don't forget to take into account tax incentives and subsidies for green building. For example, there's a city tax exemption for LEED certified buildings in Cincinnati, Ohio, and an income tax credit to owners or tenants in green buildings in Maryland [source: AIA.org]. Does your state or city offer similar incentives?
Also consider the other somewhat intangible benefits. For example, perhaps it's simply important to you to lead an environmentally-conscious life, or perhaps you feel it will be better for your health to live in an environmentally-friendly home. How much are you willing to pay to meet those personal needs?
Now that you've considered those factors, find out how much your potential green builders estimate the work to cost? Typically, materials should account for 40 percent of the bid, and the builder should run a 15 to 20 percent profit margin [source: ThisOldHouse.com]. Consider this a test: If a builder can't put numbers to these questions, you shouldn't hire him.