You have to understand Juventus has a lot of money, and they have been champions for 500 years in a row now or something, but just because they do, doesn't mean they have unlimited amount of money
I'm not saying that Juventus haven't a lot of money, Real and Barca and Manchester has a lot more.
Cr7 costs now around 60m€ with taxes/year at Juventus, when he will leave in 2 years or earlier Juventus will have 60m€ "free to spend".
The example with Mbappè, he can leave in 2 years exactly when the contract of CR7 will expire.
Juventus can use the new law in Italy and pay only taxes in 50% of salary for workers from EU, so... (easy math salary+25%= total fee)
TDLR
Mbappè can leave for free in 24 months and if he will go he will ask a huge salary cause he has no cost for the team for the sale.
Juventus we will have 60m€ free from the ending of the CR7 contract.
Juventus if wants can convince Mbappè with this money 48m € to the player and 12m€ of taxes.
Can any other team pay 48+48=96m year in salary? probably not (only in Italy we have this huge discount on taxes)