We have had two dogs and IME, they are expensive their first year and last year, and not so much in-between.
I have spent around $1000 the first year we had each of our dogs to get them set on adoption fees, shots, dog training, boarding her before we were comfortable bringing her to people's houses, equipment, etc.
But, then, I only take them to the vet once every 3 years for rabies and whatever other shots I deem needed. I decline to do any of the blood tests or other tests unless they're having a problem. We just do required shots and a checkup for around $200 every 3 years. I order their flea/tick meds and heartworm stuff online, buy the xlarge flea stuff and give the exact proper dose using a syringe, only give it to them for the required months ( see ,http://terriermandotcom.blogspot.com/2008/05/billion-dollar-heartworm-scam.html to determine when this is for where you live) and wind up spending about $50 total for the year on that stuff. So, for most of the dogs life I'm spending a total of $350 every 3 years for vet stuff, so not much more than $100 a year. We buy the tractor supply 50lb food for less than $20 a month for a large dog, and give small bits of people food (eg., cheese) for treats.
The last year of life for our last dog was a bit more expensive. He had congestive heart failure, and while we opted not to put him on drugs (which might have prolonged the life of a 16 yr old incontinent dog another year), we did pay for xrays etc to diagnose the problem. If I had another dog his age, now that I saw how useless the diagnosis was, I probably wouldn't opt for the xrays again. But, between the xrays, getting his teeth cleaned and a growth removed while he was under for them, and the final cost to put him down, we spent around another $1000 his last year.
I love having dogs and to me, the total cost of owning of, say $500-600 a year (adding up vet bills and averaging over the years, total start-up and end year costs, food, etc), or about $40-50 a mo, is absolutely worth it.