Clubs like United do absolutely not need to think about resale value, age FFP/P&S when it comes to simply improving one specific position on the pitch.
If they paid £60 mill and he plays better than De Gea has been recently for the next 3 or 4 season they won’t bat an eyelid and they would consider it hugely well spent, whether he is technically worth that or not. At the top end you pay for success and are rewarded handsomely for it.
Smells like £45m plus some additional undisclosed bonuses to me, which will amount to nothing, but will give us the ability to look like we’ve sold high.