How to calculate the true cost of things

I just read this New York Times article about weighing the true impact of shopping and eating habits. In short, it looks at the ways in which eating locally is and isn't the best option from an environmental standpoint.

It was not only an interesting read, but was in line with a series of questions that I've been having lately. The questions all boil down to something I might call true cost. Some true cost questions are easier to find answers to than others.

For instance, I found an answer to the real or fake Christmas tree question in about two seconds flat. But other true cost questions aren't so easy. For instance, we have a very large and old gravity furnace in our house. Our family efficiency expert Nathan has told me that replacing the furnace would make more of a difference than getting insulation, and just yesterday when we were getting the attic insulated (we needed to get insulation to meet Berkeley efficiency standards), the insulation guy told me that both in terms of cost and efficiency, a new heater would make a huge difference.

But is the impact of manufacturing a whole new heater and ducts, putting the old one in a landfill, and properly disposing of the asbestos-covered ductwork really less than just leaving the old one as it is for now? With all those independent concerns, it's a hard question to answer confidently. My guess is that it's a matter of timing. Say, at year 10 of a new heater, the true cost balances out or something like that.

Is there a true cost website? Or does anyone know of a service which will help you figure it out? I have the same question about mattresses, cars, and a number of other items.