I'm guessing you're over 35 (as I am)...
It's crazy money, but you gotta view signings relative to the day and age they're taking place in.
Rom vs John Stones (50m)
Rom vs Sterling (45m - few years ago)
Rom vs di Maria (60m - few years ago)
Rom vs Pogba (90m)
He's still only 23, PL proven over and over again, and rarely goes 2 matches without scoring.
In a dominant United side, he'd score between 20 and 35 goals a season, every season.
Added to that, he's settled in England, and actively thrives in the PL. He's very unlikely to start harping on about his 'dream to play' at Real or Barca etc.
Long story short - in this day and age, on his current form, he's easily worth £70m.