simplex:
Current drawn by an Atmega328p with external 16 MHz Crystal = 13 mA
That sounds reasonable if you are not using any power-saving techniques. Using code similar to Nick's demo linked above, I could get a 16MHz 5V Arduino down to approx 1uA. My multimeter is not good enough to read these very low currents with any accuracy, so I cannot say if the real current was less than that. The datasheet claims 0.1uA at 3V, and I've no reason to doubt that.