We have two dogs, both about 40 lbs, adopted from shelters. One is a German shorthair mix, the other an Australian cattle dog. We don't split out their costs from our cat, but all three combined we budget $200/month. Boarding fees we consider part of our vacation costs, not pet fees, so those are on top.
Overall the dogs aren't too bad. We go through one bag of $50 dog food a month (Merrick Chicken and Rice, they don't do well on grain free) and I think heartworm is about $20/month? Not sure. They've been on frontline in the past, but we stopped using it this spring when it coincided with some odd medical issues with the cattle dog, and haven't decided if we'll do it again...they don't go to dog parks, or interact with many other dogs.
The German shorthair has had three surgeries since we got him, totaling about $1200 or so I'm guessing. He had entropion (eyelids too big, so the fur scrapes their eyes). The first surgery didn't correct one of the eyes enough, and he ripped the second surgery open the day he had his stitches out, so had to go back in. Luckily we were in a small town at the time, so we aren't charged anything extra for a Sunday morning emergency surgery, though we did have to wait until after church!
We occasionally buy them new toys or a bag of treats, but by now they've accumulated quite a few. We probably need to do training classes again, but haven't decided if we want to splurge on that at the moment.
I think their yearly vet visit was about $250-$300 combined, but we didn't need to buy any heart gaurd at that time. The few extra visits we've had (fungal infection of the skin, wounded foot from a game of fetch, digestive upset) have been $70-$200 a time, and have happened about once a year.
I guess we technically wouldn't have had to fence in our yard if we didn't have them, which could've saved us $1900. We considered that a cost of buying our home though, rather than including it in the dog costs, just how we do it.
To us, they are worth every penny. We get a lot of joy out of them.