I would probably argue that Kante, Hojbjerg and Casemiro are outliers. Clause, contract ending and just great scout work.
Fabinho, Rodri and Jorginho are a good benchmark, but Rodri had a clause in his contract and would probably be more expensive if not. Fabinho and Jorginho was probably cheaper due to coming from Ligue and Serie A. And you could probably add a bit of inflation on those prices as well.
In the list of The Atlethic, where they tried adjusting for "football inflation", the fee of Roy Keane today was just north of £90 mill. Carrick at £69 mill. Based on these figures, you could probably argue that somewhere between £65 mill and £90 mill would be fair for this type of player. Considering his age and playing for a good PL-team I would expect it to be in the north end of that interval. I have a feeling that West Ham will refuse to sell for anything below £100 mill. Maybe they will just refuse to sell period.
The worry for Man Utd is that we really do not have any great options. Lets face it, Tchouameni will have other options. So I could easily see the club beeing willing and able to pay a premium on top of what is fair.