I have an 80lb Weimaraner. He eats a fairly expensive brand of food (California Natural) because of some allergy problems. It runs about $20 for a 20lb bag of food. Some dogs will do great on cheap foods, others will have to eat more expensive foods. What another poster said about looking at eyes and coat will give you a decent idea on how well your dog is doing on the particular brand of food you’re feeding. The food plus stuffed animals and bones probably run to about $30 to $35 a month. There are also other incidental expenses like leashes, collars, bowls, beds, vaccines, etc.
The basic expenses are the easiest things to deal with and it makes owning a dog seem like a fairly cheap thing. What you have to worry about are the medical expenses that can occur very suddenly. My dog caught a nasty rotavirus last year which wound up costing close to $1000 to treat. The larger the dog, the more expensive medical costs will be.
There are some things you can do to mitigate this, though. Spay or neuter your dog, then you don’t have to worry about certain cancers. Get your dog from a responsible breeder. The expenses for something like hip dysplasia can be outrageous.
http://dogplay.com/GettingDog/checklist.html
Or you can go to a reputable rescue, where a dog’s health and temperment will have already been assessed.
Also, as other people have mentioned, invested in obedience lessons. These can be expensive but are well worth it in the long run. And socialize, socialize and socialize your dog. Take your dog/puppy everywhere you can, invite people over to your house as much as possible (particularly if you’re thinking of getting a puppy with territorial or protective instincts like a German Shepherd). IMHO, it’s probably the most important thing you can do for a puppy, much more important than all the alpha stuff you’re going to hear, and it’s free.