Difference is Van Dijk was/is probably the best CB in the PL. He was as close to a sure thing as you can get. Taking a guy from the Italian league is a lot bigger risk, you shouldn't pay as much for that.
Even 45M is more than anyone except Liverpool and Man City have ever paid for a defender. Ferdinand and Thuram were more in constant dollars of course, but they were also clearly among the best in the world.
The most expensive ever from Italy are Bonucci, Thiago Silva and Thuram.
I don't think you can say Skriniar is in that class yet.