I assume that you don't mind reactive power consumption of the circuit but want to reduce power dissipation in the series resistor and parallel regulator (Z-diode)?
They are related to surge strength and acceptable input voltage variation and thus can't be reduced in the present circuit without disavantages. I guess, it's no necessary to mention that a transformer supply for 50 mW will be very small and cheap. Alternatively, you'll possibly want to refer to a non-isolated buck converter.