I'd say Juve may have overpaid slightly.
I don't rate Asamoah as highly as most people do, so I might value him nearer 12m.
Juve also don't really need Isla, so - while his market value may be 12-15m - his value to Juve may only be 10-12m.
So, Juve have overpaid by 6-8m (20m Cr$).