> My comparison would rely on running several bitcoin networks in parallel. Of course it does not make sense economically speaking, but it allows for a back of the envelope estimate.

Mmmm, I understand you're looking for a convenient way to mathematically equate the two on a per transaction basis for comparison's sake, but its just really difficult. For example, your several bitcoin networks in parallel doesn't really exist, because separate networks running in parallel are not interoperable (Bitcoin vs Litecoin). You lose the utility of network effects, etc.

Its like comparing energy costs of bicycles and ocean freighters, but no amount of bicycles could ever tow freight across an ocean.