I don't have numbers, but my expectation is that cost is of the same order of magnitude. I don't think coffee is all that cheap. I'm thinking of all the problems I've seen with the plumbing leaking, the machine being left on and burning requiring extensive cleaning, broken pots, and so forth, all on top of the material expense. Soda doesn't have to deal with the heat, but it does have pressure, so that probably kinda balances out.