shouldn't be a mastery that lowers the mana a skill needs ?

I can tell you, that mastery will be abused, simply because champs are balanced around their manacosts.

If you need mana in lane, you should take Meditaion, it does wonder.

I'd be okay to buff b Meditation or at least give it some level scaling. But reduce mana cost would be huge imo.

That'd probably make it a lot more difficult to balance for champions with purposefully high mana costs.