Yeah, the cost is really quite high for what it does.
I think there should be a shield in the game which prevents creatures from being damaged, maybe not a shield, perhaps just temporarily.
I think he meant to type physical sources. Which I think in turn he meant as damage sources - ie, not Otyughs Devour or Paradox (instant kill), where as Firebolt and Icebolt would be considered physical; even though they are ultimately magical, they still burn (physical) or hit with cold force (physical).
Having said this, the game currently has no distinction between spell effects in terms of classification as either 1) elemental, 2) physical/magical. Well, I think there is a little bit of elemental classification going on; take for example, Holy Light - heal a creature or player for 5hp, if the creature (or player) is of the element (mark) death or darkness, deal 5 damage instead.
The idea is not the best, but it can be improved upon.
Finally, to submit an idea for the shield competition, you need an image, Rossendale.