Even the monthly bill has been erased from the equation, as most 3g-ready readers on today's market include lifetime connectivity in the purchase price.
A version of an equation explaining a shifting price level is not hard to diagram, although there is some doubt, in higher academic circles, as to whether any such equation is of real value.
But the lack of an actual carbon price still keeps the cost of existing electricity down, and thus the necessary incentives in place to make Google's cheaper-than-coal equation a reality.