It'll be interesting to see the Snapdragon X Elite next year. It won't take the performance crown but it should finally give reasonably priced ARM outside of the Apple ecosystem something in the high performance class.

As for the real costs the upper bound seems to be no more than ~30% on average for the software layers to do it so probably less than that if the hardware is built from the ground up to run x86 code only. My guess is the real difference is <10%. Still appreciable, but not something that really drives the gaps.