Can somebody explain to me...

Riot is going to go through their database sometime and realize that Miss Fortune exists. She's like that artifact you stumble on that was made by a long forgotten race of aliens.

It is a flat mana cost no matter what the level. So it starts high but at least it stays that way. I agree it is a little high though for a non ad-scaling ability.

I think it's a good way to limit her early game. Don't underestimate the slow, it let's her synergize well with any supports and let's her very easily follow up with jungler ganks.