D&D General instead of using spells slots ...

[le Redoutable]

... why doesn't one use mana points to memorize spells ( please note that you don't cast your spells à la volée )

so it is not really a mana system, because you have to memorize your spells, and the cost for each spell could be something like
cantrip ==> 1
1st Lvl ===> 4
2nd Lvl ===> 9
..
9th Lvl ===> 100

or
cantrip ===> 1
1st lvl ===> 3
2nd lvl ===> 6
...
9th lvl ===> 55

or
cantrip ===> 1
1st lvl ===> 3
2nd lvl ===> 5
3rd lvl ===> 7
...
9th lvl ===> 19