First off no one ever uses aggregate statistics like that, without minutes or performances. Secondly level of competence shown at the highest level matters. Mbappe was smashing the CL and WC.
2 of the 3 transfers you are mentioning have failed as yet to live up to the hype. Some contemplation as to why that might be and a reflection of whether these fees for youngsters are presenting value for money is required.
They are only 19 for comparison sake and has just kick started their career for around 2 season in comparison, sure you wont expect a lot of difference in terms of minutes of game.
If you insist to go into that level of detail for argument sake, lets do it again then:
Mbappe at 19
Games: 64 (29)
43 goals
21 assists
total output: 64 goals+assist in 64(29) games
Value: 180m
Sancho at 19
Games: 65 (20)
29 goals
35 assists
total output: 64 goals+assist in 65(20) games
Value: ?
Dembele at 19
Games: 55 (17)
20 goas
23 assists
total output: 43 goals+assist in 55(17) games
Value: 145m
Felix at 19
Games: 28 (6)
18 goals
9 assists
total output: 27 goals+assist in 28(6) games
Value: 113m
In terms of total game time and total output at 19, Sancho is very similar/comparable to Mbappe. In fact, can't say which one is better. They are almost identical in terms of stats, just like Messi vs Ronaldo.
In terms of living up to hype, for feck sake they are only still at the age of 20-22, hardly consider to be flop by any sense yet.
So what's your point again?