When designing new weapon enchantments, when is it appropriate to make it cost an enhancement bonus (+1, +2, etc) and when does a flat fee (like impervious) suffice?
My current reasoning is that enchantments costing an enhancement bonus should have on-hit effects. In other words, the ability should apply whenever the wielder attacks with the weapon. Otherwise, it should be a flat fee.
I think this is wrong. I think it is more correct to think of it as whether the given effect is strong enough to displace a class feature or to be a class feature.
Weaker or situational stuff gets the flat cost.