Last time I asked around about this question, the answer was surprisingly “probably not much”! When a low-power x86 chip (like those mobile chips) is idling (which is pretty much all the time if all you are doing is hosting a server on it) it consumes very little power, about the same level as an idling Pi. It is when the frequency ramps up that performance-per-watt gets noticeably worse on x86.

Edit: My personal test showed that my x86 laptop fared slightly worse than my Pi 3 in idling power (~2 watts higher it seems), but that laptop is oooooooold.