Let's look at a couple of things here
#1) We bought Morata for 20 million
#2) When we bought Dybala, he was on the last year of his contract,and 32 + 8, despite his talent, was still considered high, taking everything into consideration
#3) IF you want to use the assumption that we would have paid 30 million to keep him, then we would have spent a total of 50 million on him, because if that is indeed true, then that is what we valued him at. And if that is the case, then we will ahve to spend at least 50 million to find a player of his age, skillset, and cache at the age of 23.