I make them do a 1hp cost on even the low powered spells. I removed spellburn. Instead each time the player successfully casts a spell a counter goes up by 1. If the player rolls that on their magic attempt then they get the mercurial rolls. So it starts at 1, then 1 or 2 then 1,2 or 3 etc.
I like the idea of the player pushing their luck each time they choose to use magic. It really adds to the tension of the magic system