Suggestion; add something they need to have to cast spells. Plain gold would be fine in my opinion, not that it would consume gold to cast, but that it simply must be in your pack, like 1k for Light 5k for Death Flame and 10k for Flame Whip, that way if they don't die its still "free" to cast but then they die you get something.

A non-bob gm who shall remain nameless did it!