Cats are cheaper to feed than dogs in my experience. Our staffy ate a 700g tin of food per day so about $1.20 plus some dry food. With cats you have cheaper food (4.50 a box of dry which lasts a few days or more) but you do ahve to pay for kitty litter too. You also have to factor in injections/flea stuff etc.
I prefer cats but thats just me, I hate having dog poo all over the yard!