How would you know? First off, no one had any idea either Cristiano, and to a lesser extent Messi were going to become as good as they became. Messi's talent became clear at 19 years old when he scored a hat-trick at the Bernabeu, but there were still concerns about his muscle injuries preventing him from being on the field and realizing his potential. Cristiano was nowhere near as good as Mbappe at 18, I'd say Mbappe at 18 is better than Cristiano was all the way up until 22+.
When a young player emerges, it divides people into the overly-cautious group or the muppet-train group. I fall in the latter. I've seen talented youngsters before. Benzema is the closest comparison I can make to Mbappe. I remember when he scored his goal against Manchester United, people went crazy for him and wanted him at all costs. I think Mbappe is even better than Benzema (who I think is more of a link-up striker than a finish-at-all-costs striker like Mbappe).
The reason I wouldn't worry about spending money on Mbappe is that he's nowhere near the finished product but he's already got so many important aspects to his game already mastered or close to mastered that WILL result in goals. His off-the-ball movement is one of the best I've seen from ANY striker, let alone a teenager. His goal against Juventus is my favorite of the season because it shows how even though he's not at his physical peak being so young, his intelligence, movement, and anticipation help him score against one of the best defense in the history of the Champion's League. He scored a similar goal against City, and has done it in Ligue 1. These are not lucky goals, nor are they characteristics that I can see go away anytime soon. Martial's second season at Manchester has been a bust, but Martial never had those characteristics. Martial is a good dribbler, has a decent finish, but doesn't have the movement of a striker like Mbappe, nowhere even close, and I'm not sure he's a very intelligent footballer yet (but neither was Cristiano until later in his career, so who knows?).