Microlite20 is aimed at being a low-magic (and therefore low-maintenance) rules set. The trade-off is that a Mage can cast lots of low-level spells (compared to a core D&D Wizard), but fewer high-level spells. This should be suitable for most styles of play, but if you prefer Magi to be more flexible at higher levels then change the spell costs by shifting them to the right.

Spell Level	0	1	2	3	4	5	6	7	8	9
HP Cost 	*	1	3	5	7	9	11	13	15	17

In effect, the HP cost becomes double Spell level -1. Allow (3+MIND bonus) 0-level spells to be cast for free per day, after that the cost is 1HP per spell.