Measuring "Greeness" of a Company

I was on a drive to a meeting when my colleague mentioned that Mercedes has just revealed its new car. The conversation develops to why Mercedes is able to sell a car at higher price. At the same level of technology (i.e. less emission per mile driven) or better Mercedes sell the car at higher price. This means to reach the same income level, Mercedes only needs to sell fewer cars than Toyota or Honda. If the car is driven for the same distance then emission per monetary unit paid to get the car is less for Mercedes. It seems the Mercedes is greener than the other two.

Let's assume that the amount of material for manufacturing one car is almost the same. With GEMIS software one can assume the emission of 1 kg of steel and any other material to build a car. If two cars from different manufacturers weight almost the same and use almost the same amount of material then the emission to produce the car will be almost the same. If there is a price difference then the expensive one will be more environmentally friendly per monetary unit of price. 

Does it mean that the more expensive stuffs are more environmentally friendly (at least from the emission parameter)? I guess the rough theory must be further proved. Not all expensive stuff are environmentally friendly for example gold. One has to dig the whole mountain and put many chemicals for purification to get gold. I guess gold is not the most environmentally friendly product. 

I am not afraid that this will make producers increasing the price of their products to get a greener image. This is just a crazy idea that come to my mind yesterday.