I go to a farmer stock supply center like "Peavy Mart" here in Canada, cause they have floating water heaters their meant for cattle troughs and they are pretty darn tough as they are built to withstand cattle bumping them with their big heads and I find they are cheaper than anywhere else at 25 dollars. The red cattle trough heater that I use is 1500 watts and that works good for me with three big connecting ponds and a total of roughly 3500 gallons. Your weather is milder you will make out cheaper in the long run with a smaller voltage heater like 1200 watts. Then I only plug my pond heater in when the temperature is below minus 10 c or roughly 16 f. If your pond is smaller, smaller bodies of water cool off quicker so you may just have to judge yourself how much you need to plug in. I don't leave it plugged in all the time as that draws electric power even when not heating much.
So you find a floating heater and just plug into a GFI circuit with a extension cord and place floating heater where there is a water flow or movement so the pond is evenly heated. I also leave my pumps run all winter but lots of people use a bubbler, which ever you feel more comfortable using will create water movement.
How much it costs to run depends on how much you pay for electric power, how much you use it and the volt you run. I don't pay much for electric power compared to many other places as Medicine Hat has it's own natural gas power plant. My husband tells me just now that he thinks we pay an extra 25 dollars a month on the coldest month, at the most.