What is the true cost of owning a dog?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in Saturday's paper. This week's question I have reluctantly agreed to get a dog after five years of badgering by our children, but am concerned at the cost. Are some mutts cheaper to keep than others? How much does food cost and what other less obvious expenses are likely to crop up. Also, where do we get it? All tips/advice gratefully received. What are your thoughts?