Lets point out a few things:
-Cassano is 5 years older than Giovinco.
-Cassano hates Juve, whilst Giovinco loves Juve. This has far too many implications to mention. (devotion in time, interaction with players, coaches, etc and motivation for the team's win)
-Cassano has great attitude issues, he wants/demands to be THE star of the team, where Giovinco seems to be willing to earn that status and respect his teammates! Cassano is not and will never be a professional player, thats why he will always fail in serious teams!
-Giovinco is an apt replacement to Diego and we need such a player more than we need someone who can do Amauri's, DP's and Iaquinta's job.
Both Giovinco and Cassano cant be the prolific CFs like Trezequet is (was).
But at least Giovinco is young (can still be converted) and versatile enough to serve as Diego's replacement.
If we give him away, we will have to buy a new player for that role and still have no prolific CF, like Treze (Amauri is NOT like that) Thus multiplying our needs without adding smth we dont already have (a goal scorer)
-ATM Giovinco is a young (rising star) sub, into a great team, whilst Cassano is the "mature" (established? lol) star, of an average team, where he can shine more.
For that reason, Cassano (who also comes from a great season) is overvalued and Giovinco is undervalued, we are most likely to pay a difference to swap Giovinco with Cassano.
Considering the above arguments, such a difference is not justified!
And if the 5 year younger Giovinco blooms, then we are the ones who should be asking for a difference!