Given that -1 refresh for two minor consequences for spell casting is probably reasonable, and assuming conviction 4:
Base
Slots: 4, 5, 6, 7
Minor consequence:
Slots: 5 or +2 to an existing slot
Opens you up to invokes
Takes at least twice as long to refresh when compared to stress.
First extra stress box:
Slot: 8
Second extra stress box:
Slot: 9
I think the argument can be made that the Invoke and 'start healing' down sides of consequences are more than enough to make up for the apparent added flexibility, so an extra stress box is worth on the order of 6 minor consequences (refresh every scene rather than every other scene) and should be priced as -2 refresh per box at minimum, no more than 3 boxs, and they still look cheap at that pricing.