I'm going to try and set up a sustained spell system once the CK comes out - cast once to activate spell, cast again to deactivate, with a fixed magicka reduction instead of an upfront cost. A longer duration for candlelight helps, but is annoying if you're a stealthy character and you're left stuck having to wait for it to wear off.
I've had similar thoughts.

This should be dead-simple, really... turn the spell effect into a script trigger which adds a constant effect (light and magicka drain) if you don't have it, or removes it if you do. There should still be an up-front cost, of course, otherwise this would wind up pretty abusive with the Alteration shield spells.