I've replaced DR/Magic with DR/+1/4HD Magic.
It doesn't seem right that a +1 sword is equally effective at bypassing DR of a young adult dragons or ancient dragons. Or a CR4 Gargoyle vs. a CR13 Azruverda.
I've also changed the cost for special material from a static value to a base price multiplier: mithral x15, adamantine x30, etc.