My half dog just turned two in September. She has certainly become a member of the family and even has her own category on my budget. But the reason she has her own category is because she does cost me money!
Since our dog is still relatively young and hasn't incurred massive vet bills, this is by far our biggest expense. At first I bought her food at a local discounter for a pretty good deal. It was about £15 for a 40 lb bag. Problem was, our dog would not go near the food. She flat out refused to eat cheap dog food. She was losing weight so I caved in and bought the stuff her vet recommended. Now I pay about £30 for a 30lb bag.
Most people assume they will just wing it and figure out a place where their dog can sleep and stay when you aren't at home. From my experience, this is where best of intentions go out the window. We bought a huge crate for about £50. Problem was when she was a puppy it was so big that she could do her business at one end and sleep at the other, thus prolonging potty training. That's why we now have two crates, one big and one small that are never slept in by our dog. She now prefers our king size bed.
Also, another thing we had to invest in was a fence outside. This is obviously a huge expense and will vary from home to home. But don't underestimate what you will need for your pet. I assumed I could just take the dog out on a leash everytime, thus avoiding the expense. Needless to say we were constructing a fence after about a month with the dog. Total Cost approx: £1,200
This is where your costs will vary widely depending on breed and age. I would look into pet insurance if you want consistent expenses. So far we have probably paid about £500 in vet bills. Luckily we can afford it, but I know some that have gone into debt with vet bills and now are working with Debt Free Direct.