@golden_blunder is absolutely correct. From an accounting sense, whatever the incremental cost you think United are overpaying for Maguire would just be amortized over the life of his contract. So, paying an extra £10m (on what you think is a fair price) is really not a big deal if he's staying for 5+ years. I would argue wages are much more important.
Ignoring accounting he's still correct. If he's the player United/OGS want and he fits within the budget why does it matter? Why potentially lose a player you think will be important for years to come over such a small cost? You aren't buying a stock on an exchange. You're trying to build a winning side.
Nobody is asking or bidding £100 for Maguire so that part is irrelevant.