Cost per tokens is as valid as price per unit volume of fuel.

Changing the fuel type, efficiency of your vehicle, driving distance, or driving conditions will all change how much it will cost you.

Fuel cost per unit volume does not become meaningless just because you are neglecting all of the other factors involved. That would be throwing away the only data point you have been using.

This is just asking for someone to amalgamate all of the factors involved into one simple, easy to game, index.