I think usually the fans would be pissed if a player is brought for such a huge amount because then they can't brag that a player was brought at a low price to others, or the fact that the rivals will taunt that United spent 75 million on Di Maria. If United win the league then it would be that United brought it rather than win it.
I could be wrong but other than the factors above, there's no reason why fans should be bothered about how much money is being spent on players, but the emphasis should be if the player coming in would instantly improve the team. In case of Di Maria and Vidal they would improve the current team by a good amount.