Yeah, Costa, Depay, Schweinsteiger, Maria, they cost a fortune yet contributed nearly 0. Some world stars those turned out to be. Often these guys come from a team that was build around them, like RvP during his time at Arsenal, and it just doesn't work in another system.

Then again, De Gea, Blind, Martial, Mata, were all great additions, and definitely world class. Perhaps you just can't predict how it'll turn out. But having 10 world class player on your team will play out differently than having 1 and a decent team. In the latter case he shines, in the previous one there can be bitterness, jealousy, etc. I see that a lot when Depay plays, since he always wants to show off, to show that he can be worth something, and that is something you don't do in the BPL.