I don't think turning down this sort of money is nearly as surprising as some think. Look at it this way (this isn't specifically about Higuain, but any top player):
Say you can earn £10m a year playing for a top European side, or £40m a year in China. Either way you are unbelievably, preposterously rich. So what do you spend all that money on? Some houses, some cars... yeah sure, no problem. But what else enhances your life? Living somewhere you want to live? The chance to to get something that money literally can't buy... the glory of winning trophies in Europe's top leagues?
Or to put it another way... if money was no object, and it was possible to buy yourself the chance to win the Champions League with United, how much would you pay? Is it that mad for a guy who has well over a hundred million pounds to his name to pay tens of millions for that opportunity?