More often, though, such a balance comes out as the energy needed to do the magic is actually drained from elsewhere, be it the caster or the caster's assistant or the land or spirit realm or what-have-you. Just as one example, the mages in Guy Gavriel Kay's Fionavar Tapestry use "anchors," people who are bound to them magically and who act as their strength and source of power. Use of magic drains that person, much like physical labor wears you out.

::nods:: yeah, I've read alot of stories like that, though I'm curious to see if there's actually more stories where, say, healing causes the equal and opposite reaction of wishing for destruction in the healer themselves.