Getting a purebred dog can be expensive, sometimes absurdly so. Adopting a dog from an animal rescue is generally somewhere around $100.
Once you have the dog, expenses are:
Food - maybe $50 a month for a bigger dog. Depends how much they eat and how nicely you want to feed them
Vet Care - maybe $200-$300 a year
Boarding or sitting when you are out of town ($30 a night)
And then the bigger expenses:
Carpet when the dog tears it up
Extra vet visit after the dog eats a diaper out of the trash can
Hypoallergenic good because the dog has a sensitive stomach
Obedience school because the dog is crazy
We've got a boston terrier, and we like him, but he is expensive.