I was concerned about my electricity costs, but I think they're relatively minimal I've got this equipment running on five tanks ranging from 5g up to 40g:
Four 7W AquaClear motors - on all day
One Eclipse motor of unknown wattage, probably less than 7W - on all day
650W worth of heaters - on intermittently as needed
236 W of fluorescent/LED
light - on ~10 hours a day
Generously figuring that the Eclipse motor is 7W and the heaters are on 12 hours a day, I'm using 11kWh at a rate of about $0.15 per kWh, so my tanks are costing me $1.65 a day in electricity to run. That seems cheap to me, but I can't find an error in my math.
I made an effort to do smaller PWCs to cut cost, but I haven't really noticed any significant drop in my water bill. I figure I change 30-50g of water every week, but that's a small fraction of what the toilet and the shower use.
The cost in fish keeping is the buy-in. Once you've got the tank set up, it's pretty cheap to maintain unless you buy a lot of live feeders. Compared to buying food and vet visits for a dog, fish are pretty cheap.