Typical "portable ACs" with a single flexible hose are inefficient for serveral reasons.
Agreed, I use this method

with the tube up the chimney, but this should show more not less cost using portable to fixed AC's.
I looked at mine, in 755 watts, out 900 watts, and to be frank most of the power seems to be in running the fans. It will keep the room cool, but if room allowed to heat up, not really good enough to cool it again.
I think the costs quoted are well out. I had this with immersion heater use, since on a fused connection unit, not easy to plug in an energy monitor, so I just believed what I had been told. I now have an iboost+ which tells me how much used, 50 kWh per month, (28 days) summer only as winter can't stop central heating also heating the DHW.
So I was running the central heating boiler in summer 4 times a week ½ hour at a time which was really not enough, Boiler is 20 kW, so it has worked out cheaper to use immersion heater, and more convenient, and water temperature better controlled. Even when, it only runs when the sun shines. 160 kWh using oil, and 50 kWh using electric per 28 days, I had just believed what I had been told.