As much as I tend to criticise teams who go out and just sign any available good player, regardless of what they already have, I'm not sure that approach will prove too problematic for Man Utd just now.
That's because they basically don't have a proper team. They have so few genuinely good players that they can just go and sign whoever is available and then figure out how to fill in the other gaps in future transfer windows once they get close to having enough good players in their team/squad to enable them to do something.
Still, you can see how various factors have forced the price up (Man Utd's desperation, January window, buying from a rival) because I really can't see how you can say Mata is worth more than £30m (and that's me being somewhat generous).