I'm pretty sure whoever signs Milik for "free" will have to pay him a fee. It's like this with all these high profile bosman transfers, Milik won't be the exception.
Scamacca I don't see necessarily why he would come and earn more than Buffon, Demiral, and Pirlo himself. 2 or 3m is too much, he would be cheaper.
We are talking about a more than 20m difference in costs between the two.
Better these two anyway than Pelle, Llorente, or most other names for this backup role. It is very different though to sign Milik than to sign a young player. Milik demands playing time and the salary of an important player, which he is.
IMO better to not to invest in Milik, better to sign a cheap backup striker and take that money to Sassuolo for Locatelli.