In last decade and even longer, Manchester United was completely a newbie on transfer market by purchasing players with super overvalued and expensive transfer costs. It is more painful as later they did not use those expensive players well enough and made it like doubled failures.
All clubs can make transfer mistakes that barely happen while with Manchester United, they have been doing this in many latest seasons. Hence, it is no longer mistakes but more likely severe systematic problem from their club strategies for development and necessary actions on transfer market.
anything can happen and mistakes are always there, so is Manchester United, many players are bought at high prices but when they sell them back at prices that are not comparable, however, Manchester United does focus on players who have advantages that they consider worthy of joining their club without looking at the nominal offered. even though the transfer market value of the player is high, they do not see it as an obstacle and that is part of the strategy used.