Depends, on many things like insulation, ambient temps/conditions, etc. My aq heater draws 50watts, deicers are usually 250W....but measuring power consumption is necessary to know
just how much power a heating device is using.
I tracked my power usage with the aq heater during a frigid winter with a Kill-A-Watt device and recording daily readings over 3-4 months. Calculated that it cost about $2-3 a month on average over the winter...I thought that was reasonable for the worry free ease it provided.
You definitely have to keep water topped off and protect heater and vessel it is inside.
I built a 'cage' around heater and add only warm water daily, easy to carry out a gallon jug each day during chore time. I suppose it wouldn't be good for those not doing daily feeding, watering, observing.
https://www.backyardchickens.com/articles/aarts-heated-waterer-with-horizontal-nipples.67256/