Question for 3d hubbers

[curious aardvark]

Currently setting up my hub.
So most people seem to charge by the cubic centimetre.

couple of questions.
1) cubic centimetre of the model (including spaces) or the amount of plastic used ?
2) how do you work this out ?
Simplify3d gives me cost, amount of plastic in grams and length of filament.

I don't honestly think I've ever seen a cm3 setting in anything.

Why is such a bizarre measurement used ?
And how do you work out how much to charge per cm3 ?

It strikes me this setup could have been a helluva lot easier without such a weird way of charging.