It's partly because we've been playing crap football since Fergie retired so the value of our players isn't as high, most of the time they look worse when they join us. When they're in an attractive attacking system they have the opportunity to really showcase their value, thus we can demand more money for them. Plus when a player does want to leave, we offer crazy contracts and beg them to stay anyway. Another big part of it is simply crap negotiating. Remember the huge exodus in players under LVG? We sold RVP and Nani for about £8m combined ffs.
Di Maria still had value because it had only been a season since he moved, plus he still produced a decent amount of assists. Lukaku is similar in that he still was our top scorer and boasted a very good PL goal scoring record. Still lost money on both though. Schneiderlin's the only real surprise, and I'd put that down to the boost in PL money overinflating value.
Right now the only player that I could see leaving for a large amount of money is Pogba, and that's not because of his performances with us. If he was performing to his standard at United, we'd be getting near 200m valuations.
Liverpool for example managed to sell Coutinho for a crazy price because they were able to create a team that allowed him to flourish. With Liverpool being a lesser club at the time, it allowed the better teams like Barcelona to take him away. They also negotiate very good prices for their players, they get crazy amounts of money for shit or unwanted players.
