Depends on the size of the dog. Smaller dogs are comparable, bigger dogs much more expensive. Big dogs just need more food.

Haven't seen prices recently because it's been a while since my cat died, and it was years before my dog died after that (though I still miss them both way more than is fair), but the cost per ounce of food for each of them was roughly the same, a difference of pennies on any given brand, and it wasn't always one that was the higher.

My dog was a corgi, about 28 lbs, my cat was something like 10 pounds (iirc, it has been a decade, and I'm not digging for her records just for this). At one point, they were both on the same brand of food, and the cost was comparable to their size difference. The dog was more expensive, but when you break it down per weight unit of food, it was negligible difference.

Just be aware that you can't always get a direct equivalence. Using purina as an example name, you might have purina one for cats, but there's three or four versions named different from the dog versions that are similar. I'm not saying that's the case with that brand, just using it as an example for what I'm talking about. When there's multiple varieties of a brand, figuring out which versions are the closest between cat and dog food isn't always obvious.

If you don't know how to calculate that, lemme know and I'll type it out for you (no bullshit, not a snide thing, not everyone knows how, and it's not a bad thing to not know).

Expect to pay more per year for a dog the size of a corgi compared to even big cats like a Maine coon. But the difference per pound of food is maybe 25 cents at the high end of difference, which is not much per ounce at all. If it's a smaller dog like a chihuahua it might end up being less per year than the average cat.

I do remember a patient of mine that had great danes. Back in the nineties, for each dog, he was paying around 300 dollars a year, and he was using fairly cheap food, plus supplementing it with his homemade stuff. It would be something closer to 500 a year now, if I'm remembering the inflation correctly.

southsamurai has a great overall explanation. I would add it also depends on the age and any medical conditions of each.

We have a 45 pound dog (age 12) and a 15 pound cat (age 17). The dog is on senior/old man food but is otherwise in good health. The cat has kidney disease so we have to get only specific kinds. Per month the cat’s food is about $5 more than the dog’s, but that’s for a smaller amount.

Depends if you want your cat to get chronic kidney disease and or diabetes. If you don't care about that, then dry food is exceptionally inexpensive... Although if you end up caring about them in the end, the special food you will need once they have kidney problems will far out way the savings.

Cats should be eating almost all wet food.