Optional rule for spell casting characters

The first thing I want to do is present an optional rule concerning spell casters. Right now they make a saving throw, and if they fail they drop one condition level cumulatively until they pass out. This optional rule does the same thing, but in a slightly different format.

Rule:
Casting a spell of any kind costs Hit Points. The cost is 1 + double the level of the spell being cast: This only happens on a failed save, and is non lethal damage only. This causes all spell casters to lose non lethal hit points until they actually pass out from lack of energy. It works well, and is only meant as an option to those who may not want to use the rules as written.