Hi @Doug Danoff - glad you're posing these questions. As a rabid capitalist, and an even more rabid, foaming-at-the-mouth environmentalist, I think about this stuff frequently. Math is quite helpful here, if you can bear some assumptions.
Let's assume the typical $700,000 house in Boston costs $5,000/year in utility bills. Let's assume that if you spend $80,000 now on geothermal and solar panels, you'll save $5,000 per year on utility bills forever, but if you spend $15,000 on regular heating and cooling equipment, you'll have a typical annual cost of $5,000 in utility bills. (note- this is rife with assumptions already. Geothermal and solar are cheaper to maintain, which is a big plus in their column, and will the cost of electric/gas/oil rise or decline over time?)
One can consider this case similar to a perpetuity, which is an annuity that pays out forever. There's a simple equation to determine the value of a perpetuity, it goes like this:
Present Value = (dividend per period)/(discount rate)
More concisely: PV = D/r
If the dividend(in this case, $5,000 annually) is divided by the discount rate and the present value is greater than what you spent to install the system, then congratulations! It made sense to make the investment- so long as you can convince a buyer of this, of course. In the above example, an extra $65,000 was spent on geothermal and solar to yield a $5,000 annual payout(really it's divided over 12 months, but let's keep things simple, if it's not too late for that.) Assuming a 6% discount rate, the calculation looks like:
PV = $5,000/.06 = $83,333.
Since the present value exceeded the investment by $83k - $65k = $18k, it looks like a savvy investment! What discount rate would make this a break-even proposition? $5,000/$65,000 = 7.7%. For discount rates higher than that, the proposition would be a money-loser.
But I think you'd have a hard time convincing a buyer to pay an extra $65k for the above, regardless of their discount rate. There are air-source heat pumps that work very well down to zero degrees Fahrenheit, and some work well even into sub-zero temps. I think a good economic argument could be made for an air-source heat pump with a natural gas furnace to provide backup below certain temperatures. If you wanted to get really freaky, you could try to come up with an equation dictating the outdoor temperature at which the furnace kicks in based on the ratio of natural gas prices to electricity prices, taking into consideration the variation in efficiency of the heat pump relative to temperatures(but that would be pretty challenging! Not only the equation, but programming that equation into the system so it adjusts the starting temperature according to the current gas and electric rates.)
Before doing any of that, I'd make sure the low-hanging fruit has already been picked- LED lighting, water-efficient fixtures(sinks, toilets, shower heads,) possibly spray-foam insulation to reduce drafts, to name a few.
MG