Food. You can't get away with feeding the cheapest food you can find because your pet is "just a dog." You won't have a healthy dog if basic nutrition requirements are not met. A dog who isn't in good health is far more likely to get sick and cost you a heck of a lot more in vet bills down the line. Take a look a dog food prices and read labels before you adopt. Get a general idea of how much money will need to be spent on a weekly or monthly basis. Obviously a larger dog will require more food than a smaller dog, and food will cost more money. A good commercial food is just fine. Dog food companies really do want the best for your dog. After all, they wouldn't be in business long if dogs were dying due to eating it. Avoid foods with a lot of cheap grain fillers like corn. There is little nutritional value in those foods, and while they will fill up your dog's stomach, most of it will go right through him in the form of a lot more waste for you to clean up. If you can't fit the cost of dog food into your budget, you shouldn't have a dog.
Sanitary conditions. It's really easy to forget that dogs like to be clean - just like people. It's very important to clean up their waste regularly whether it's a home, on the street or at a park. Your neighbors will not appreciate having to clean up your dog's waste when it appears on their property, so keep cleanup bags handy when you're on a walk. If you physically can't do that yourself, then add the cost of hiring someone to do it for you. If your dog has a short coat requiring only occasional brushing, count yourself lucky! Many dogs must have daily brushing to maintain skin health along with regular grooming. Check out groomers in your area for pricing before you adopt that afghan hound or collie. All dogs need shampooing regardless. Dogs, like many hounds, have very oily skin that can harbor bacteria resulting in a very smelly dog. Such dogs will need a bath at least once a week. If you can't afford a groomer or don't have the time to take care of your dog's coat, you shouldn't have a dog.
Living Conditions. As the result of domestic breeding over centuries, your dog is not capable of living in on its own in the wild - at least, not for long. Those necessary life skills have been bred out of domestic dogs to allow them to live with humans. Many dogs should not spend their lives outside in either extreme heat or extreme cold. If you aren't prepared to train your dog for inside life, don't adopt a dog. They are very social pack animals and you, the human, are inviting a dog into your pack. If their social needs are not met and they are primarily left outside day and night, you will have an unsocialized, neurotic dog who will be a nuisance at best and a danger to humans and other animals at worst. Domestic dogs still retain enough DNA and instinct from their wild forebears to revert to some of those behaviors if they aren't properly socialized to both humans and other animals, which brings us to...
Training. Dogs have to be taught the house rules if you are to have a successful and meaningful relationship with him. If you're new at dog ownership and have never taken a dog to obedience school, that's another cost to consider. Training is all about teaching humans to effectively communicate with their dogs using positive reinforcement of desired behavior. It's not at all difficult and can be a tremendous amount of fun for both of you. Training doesn't stop at the schoolroom door. If you don't have the money for training or the time to do the homework, you shouldn't have a dog.
Exercise. EVERY dog, no matter how small or large needs both physical and mental exercise. Wild and feral domesticated dogs are hardwired to travel for food. They aren't meant to be couch potatoes. Many behavioral issues stem from dogs not getting enough structured exercise. They get frustrated and neurotic and will eventually find their own exercise and entertainment methods, most of which make their human pack members very upset. Simply having access to a large fenced yard is not exercise. Learn how to properly walk your dog keeping him at your side and focused on you rather than every squirrel that crosses his path. Playing catch is great exercise for dogs with agility talents. Hiding a toy with a treat inside is a great psychological exercise for dogs with tracking genes. Herding dogs like to play "soccer" using their snouts to score goals. If you have a small dog, avoid the urge to carry him around in your arms or worse, in a bag. They may be small, but they're still dogs and they need to get around under their own steam. If you can't mean a dog's exercise requirements, you shouldn't have a dog.
General Responsibilities. Do not rely on your children to take care of your dog's needs. While they should be involved at every stage of care including training, they are still children. YOU are the adult and ultimately responsible. If your unhappy and unsocialized dog should cause injury to a child or adult, you are the one who will be held liable for the behavior. If you don't have the time or energy to take responibility for your dog's care and training, you shouldn't have a dog.
So there you have the basics of dog ownership. If it sounds intimidating, I meant it to be. Bringing a dog into your family is just as big a decision as adopting a child. Sometimes the expenses of owning and caring for a dog can be almost as much as a child. If you enter into adopting a dog with your eyes wide open and as an educated potential owner, you and your dog are far more likely to have a long, successful and extremely enjoyable relationship. After all, isn't that why we take dogs into our lives? :-)