Mbappe is 18 years old and looks to be Henry’s heir in terms of the amount of goals he’ll score and create. If he manages to meet his potential he’ll be worth it because he will give any club he plays for a good 15 years of football. If they go on to sell him in future, even if he doesn’t meet his full potential but still ends up a very good player, he’s still going to be worth 80m+ given how prices keep increasing year on year.
Rio Ferdinand was seen as a crazy buy by United when we signed him for 30m back in a day when nobody paid that amount however it ended up being extremely worth it, both because we got a world class defender for a decade but also because 30m is peanuts in the grand scheme of things (though it was a large amount at the time).
One just needs to realize that money in football has lost it’s value – when a Championship team can ask for more for one player than other clubs at their level are worth in total, you know that all sense of scale has gone overboard.