That is not what I am discussing. My claim is that in the current world we live in, given that I am going to make an online purchase, and I get to choose between spending £300 via visa giving the banking system £15 or spending £300 via monero giving the monero system 0.3p, I think the climate would be better off with me going for monero.We are discussing the unavoidable electricity cost and hence CO2 emissions of running the system. Whether a company runs at a huge profit (visa) or a huge loss (monero) really is a distracting irrelevance to that.
Given that the cost of editing a number in an electronic record is not dependent on the magnitude of the number, one transaction per day (i.e. 365 transactions) will have about 365 times the functional processing costs and emissions.
We know my 0.3p pays for ~1/30th of a kWh. If we assign environmental cost according to money visa needs to have an energy efficiency of 5000x that by turnover, which means releasing less than 1 kWh's worth of CO2 for every $500 they take in. This is not credible to me, I think it would require them to be by far the most energy efficient business in the world. I do not think this is your argument.
Of course my logic of the environmental cost following the money is not the only way of looking at it, but it makes a whole lot more sense that "the actual cost of changing the value in the computer" if our objective is to minimise our CO2 footprint.