The last video from "hardware haven" I saw (not the last released, just the last I saw) found:
Fuzzy memory on details: a 5th or 6th gen Intel idled at 7 watts vs an ultra efficient at 5 watts. He calculated out that it would take 2-4 years, depending on your electricity, to pay for the cost difference of a new ultra low power machine. CPUs and even graphic cards have gotten much better at idling very low.