I have never tried to figure out what one tank heater costs to run.
In the summer we have a few 500 gallon tanks that we use to water livestock, they would need to be filled every few days, in order to keep the same tank open in the winter it would require at least one 1500 watt heater. Then we switched to-- for winter---a 100 gallon tank instead; as we can use two 250 watt heaters in it to keep it open, but requires filling each day. By using 2 smaller heaters instead one larger one we cut down on the power bill.
So depending on how much water you are trying to keep open- will depend on the size of heater needed.
We use several tank heaters and have heated water dishes for most of our birds, our electric bill goes up about $600 a month when we plug them in. For our ducks & geese we have ponds which we leave a trickle of water on running thru them 24/7 to keep them open, as well as our cattle & horse water tanks. I am hoping this next summer to route the cattle water tanks over flow into some of the duck ponds so we can get double the use out of the running water and have more flow thru the ponds.