It’s non-sensical to base the cost effectiveness of a heat pump on the handful of really cold days when it’s no more efficient than electric resistive heating. You have to take into account the entire heating season. Electric resistive heating is allways 100% efficient. Air sourced heat pump is 100% efficient in the worst possible conditions. In normal conditions it’s 3 to 5 times more efficient. While your 650 watt space heater puts out heat at the constant rate of 650 watts, a heat pump outputs 3000 watts worth of heat while using that same 650 watts of energy.