Manchester united is not good in doing transfer business, they always buy players at over high price and let them go free when they are worth less. It is not only Manchester United that is in this bracket, every other clubs in England are victims of this. It was only Arsene Wenger of Arsenal that understood the business and it was going fine for him.
Manchester United are that way for two reasons, one is that the transfer market in football this days has gone abslolutely crazy, clubs now place an outrageous valuation on their players, in a bid to make sure they get the most out of them, and two because Manchester United are a deperate club, they have not won the league in a long while, they have changed managers a lot in the last couple of season, and now they are just 'ticking every box' in the hope that one works out for them, and when it doesn't, they are left with egg on their face and they sometimes have to sell the players for very less.
Having said that, you can't compare what Wenger was doing at Arsenal then, with what the situation of the market is now, if you take that approach, i'm afraid you'd not win any trophy, and would prolly struggle to even finish in the top four, this days you can't even get average players for a cheap amount, you have to spend money to stay at the top, that is the reality mate.