Thats an insane salary for someone with a track record of showing up when he fancies it. I'm all for paying players for their talents but his base salary should be far lower than that more of his salary as performance based add-ons.
Was he not happy on £200k/week? A large salary doesn't make a player happy in any long term sense and it certainly doesn't motivate them these days. Being on a much lower salary than you should be probably does but £200k/week base salary was plenty enough for someone of Rashfords talent and temperament.
That being said, football has been silly for a long time now and we have been at the cutting edge of paying players far more than they are worth for years.