It depends on the food. I think some are more calorie dense per oz. I do reviews on pet food elsewhere, and the one brand I've reviewed is Fancy Feast...it recommends 1 can per 3 lbs. of wt (twice as much for a kitten). At 33 cents (if you luck into a sale) up to 59 cents per can, that means an average 6 lb. cat will cost 66 to 1.18 per day, and I think most of us have cats that weigh a bit more
Ahem...we won't discuss Alix P. Curl's weight...lets just say when she runs, there's a lot of bellies swinging!
Plus...if you are talking a premium food, which Fancy Feast is not due to its use of by-products, artificial colorings and flavors and more (these are criteria I use), you are talking Felidae, Solid Gold, Active Life, Precise...and while they come in 6 oz. cans, they are more than twice the cost of two Fancy Feast cans.