We have both extremes in our house dog wise;
A large, young & healthy dog in a city that costs us an arm and a leg. Had I known the full extent of the costs I would've made different choices. Due to the nature of where we live we probably spend almost $900/month on him, but in a financial emergency that total would drop to 200 or less.
A small, older dog with lots of health problems. She's a sweetheart and probably costs us less than $100/mo, but it's lumpy because it's mostly vet visits.
If I could do it again I'd get another small dog before a big one because it's so much cheaper to ensure a great quality of life where we live. However, we are living proof that a large dog does better in a small apartment with love, training and exercise than hanging out alone and neglected in a large house with a great back yard.