[RogerBW]

Multiplicative modifiers make sense if the modifiers enhance each other. For attack powers, this is generally the case: Area Effect lets you attack multiple targets, Armor Divisor lets you attack tough targets, having both lets you attack multiple tough targets. I'm not sure it's necessarily the case with non-attack powers.

But of course costs are always very approximate, because campaigns vary, even in the same world. The first example that comes to mind is Mathematical Ability in a WWII game – great if the game's set at Bletchley Park, not a whole lot of use if you're doing small-unit actions in France.