[scott_w]

> My workstation runs all the time and I don't care that it consumes this 100$ of electricity a month.

That has nothing to do with zero marginal cost. The term specifically refers to the cost of producing a new copy of the software, which is so low as to be effectively zero. Almost 100% of the costs of software engineering are what would be called “fixed costs” in economics.

This is to differentiate it from the production of, say, a car, where the marginal cost of building a new Ford Focus is a significant portion of the price.