The price difference for Juventus is very important. If you get De Gea for free and sell Szczesny, of course that is an absolute advantage for juventus.
But, what is said by @Naida_BR has a point. That it is also possible for Juventus to get de gea and Szczesny to stay together, it will be even more beautiful.
Although I would like the idea of having both DeGea and Szczesny on the same team, I don't think it's as economically positive
At the moment Juventus has 4 goalkeepers
Szczesny
Buffon
Perin
Pinsoglio
Of these four goalkeepers Perin should leave in January, he has not left now because he is hospitalized from the accident.
Having said that, next year Buffon probably leaves, Pinsoglio expires in 2020 but should renew and Perin should be sold outright.
Szczesny and Pinsoglio would find themselves again and the second goalkeeper would be missing.
The value of Perin is about 15 million with a gross expense of 4. The value of Buffon is 0 with a gross expense of 2.
Having said that they would save 21 million which would be equal to DeGea's engagement for even 2 years.
Although DeGea is free but with a 4/5-year contract, it has an expense of around 100 million with taxes, it is not a small expense especially if we consider the thickness of a possible second goalkeeper like Szczesny.
In my opinion, the only possible way to take DeGea is to sell Szczesny.
You can't put 2 (expensive) roosters in the same house.