The "is he worth £100m" debate feels quite futile. He may be, he may not be. Obviously in isolation you'd want to pay 60-70m which is basically the going rate for PL-proven talent like Mbuemo or Cunha. But then you add the English and CM tax on.
Regardless, he's to my mind clearly the best and most well rounded CM on the market right now, by a small but noticeable distance. He's going to be a linchpin at CM for the next decade for somebody and for England, and is clearly a "make it happen" level talent, rather than one you try to extract value from the market on.
Looking for value in the market is why we've not had a good CM since Carrick.