I think my tank (20g planted) costs me about $20-25 or so a month, including food, water, electricity, fertilizers, water conditioner, and considering that food, water conditioner, and fertilizer lasts me months so the cost is spread out over several months, I'm not buying those things every month. The only thing that needs to be paid for every month is water and electricity, for me.
Personally, I can say our family cat is more expensive than my aquariums. A year or two ago our cat got into a fight with another cat outside, got an infection and got really sick. We had to take her to the only emergency vet in the middle of the night. The whole thing cost at least a thousand dollars to get her treated. We keep her indoors now, but then we have to pay for cat litter instead which is maybe $10 for the brand we get, plus food ($7 if you get the cheap stuff), and flea treatments (really expensive, don't remember the price) if I accidentally take fleas home from the cat shelter I work at. If it's an outdoor cat, there's a big risk of having to pay for emergency treatment. Indoor cats cost less since you won't have as much a risk as paying for emergencies, but then you have to regularly buy litter and both indoor and outdoor need food. And money for cat toys, beds, furball meds, joint medication (old cats), and vet visits are a possibility.
I'd say they're about the same price, as long as the tank isn't an expensive setup or there aren't any cat/fish emergencies. Keeping any animal is expensive, I don't know of any that don't have their costs. It's pretty easy to add up all the costs of your tank, and check the prices of things online. Perhaps you do have an expensive setup, but there's no telling until you add it all up.
Note: all the prices I put are in United States Dollars.
