> But what you won't capture is that these two goods are both providing lighting, and that the switch from one to another provides a sharp drop in the price of light that is not captured by the change in prices of either in isolation.

I wonder if there was any way to compute light emitted per source. Then you could calculate a price per unit of light and whale-oil lamps would be obliterated by incandescent light builds which would in turn be obliterated by LED light bulbs.