Quality wise, he shouldn't cost more than 50m, but that's not how the market works because if teams are prepared to pay over 70 or 80m then that's what he's worth in the end. Normally, I wouldn't pay this fee if the board won't hire a manager that will value possession above all because, as with Pique, having possession hides his biggest shortcoming: pace.
De Ligt's positioning looks good for a 19 yo kid and it can be improved even further so he does have that going for him plus leadership and good physique.