Creative help with changing basic elemental/cure spells?

I hate to say it since its kinda counter hacking but...

I'd think the "Keep it Simple Stupid" method of doing this would be to drastically raise the mp cost of said spells. You wouldn't cast Fire 3 for 200mp if Fire 2 (even with less damage) will still kill the monster. As I said, simple and stupid method, similar result, less effort. Not that I would encourage something so lacking in creativity but...