One place to look for numbers more solid than people just claiming they would pay more, could be in areas with deregulated markets for home electricity, like Texas, where people have a choice of generation provider. Since generation is decoupled from transmission and maintenance, providers don't compete on reliability, but mainly on price and environmentalism. The "green" plans are typically $0.02-$0.04/kWh more expensive than the cheapest plans. Would be interesting to know what proportion of people opt for one.

Examples of such a provider and the plans: http://www.greenmountain.com/texas-oncor