He's a good player, but he's relatively unproven having only had one really good season. As Alen has said repeatedly, he'd be a good addition for a team who already has some very good attackers, but isn't someone to base an attack around.
I'd say Rossi is worth 15-18m, so would go for about 25m in the current market.
However, while I have no particular bias for or against Italy, I am deeply sceptical of La Liga in general, so it's possible/probable that I underrate La Liga forwards in general.