I'm basing my expensive on the Asus Maximus Hero XIII & z690 Hero. Both use top of the range ALC4082 codecs, ESS® SABRE9018Q2C DAC/AMP's. Both tested on HD-660S. Both sound amazing.
My experience with Gigabyte boards in the £200 range is that their audio isolation is inferior, I've heard some electrical interference. This is likely due to less attention being made to isolate the signal on the motherboard, or possibly due to having less layers on the PCB.
Add that to the overclocking features, SP rating of CPU's, stability, XMP compatiblity, most extensive QVL list etc, explains why these are so popular and why Asus can comand such a premium.
That said, with £125 rebate (£100 from z690 promotion, £25 from review) they aren't such a high premium as it would seem.