If Pep takes him up a level he'll be worth that. He's one of the most creative players in Europe, his actual stats show that, and you'd think that they'll only get better at City if he stays considering they obviously score many more goals than Aston Villa every season. He's also 25 so is at a great age in that he won't even peak for another few years.
Considering Aston Villa don't need to sell him whatsoever, and clearly he wouldn't be upset to stay, City are paying probably 30 or 40 million more than what he should be worth. But he has the potential to be world-class. If he signed for Spurs this place would be going mental, regardless of the price.
Though there's numpties here that probably insist Ndombele is better than him.