Pogba will end up being worth the price in relitive terms, he's certainly one of the best midfielder's around and when the obsession over his price tag stops he will be reconised as such. Lukaku im truly not convinced ever will be, his goals no doubt he will score, but aside from that i can't see how he fits in.
Costa fit's us outside his goals because he's tenacious, presses well and is incredibility smart tactically with his off the ball movement. Even when he has his games were he can barely control the ball properly he gives us that. Lukaku falls short on those tactical level's compared to Costa and i fear our shape and team balance could be heavily affected by the fact Romelu is pretty lazy and hasn't got great movement.