Hi!
Our female mix is 120lbs. Males are typically larger. A full grown pure St. Bernard male can get up to 260lbs.
For us, Vet bills run around $400/year (checkup, Sentinel*, shots, etc.). We go through about 40lbs of Iams large breed food a month which is about $45-50/bag. Our Roxie has hip dysplasia so we also give her over the counter (human) supplements (glucosamine, condroitin, MSM, Omega 3, Vitamin C) which is an additional cost.
We also have a small house and the biggest problem we have is when Roxie is standing in the way of where you want to go. Unlike a little dog, you can’t always just step over or nudge her out of the way. I recommend crate training any dog, large or small. Dogs like smaller spaces (dens) and crate training makes life a lot easier for you too. Also, don’t encourage “up” behaviour when they are puppies. Keep the dog on the floor – don’t put it up on your lap or the couch, etc. That’s fine when it’s only 20lbs but when it’s over 100lbs it will be uncomfortable. 
I can’t really comment about yard size. Roxie gets tired on a short walk around the block due to her hip dysplasia so we don’t exercise as much as a “normal” dog.
Invest in obedience training and get a prong collar. Roxie has no hips but she can still pull me down the street if I don’t have control when we’re walking. The prong is a much safer solution than a choke collar.
With respect to hip dysplasia, all large breed dogs have a higher chance of hip dysplasia. Roxie was the only one in her litter of 7 that ended up having dysplasia.
Hope that helps!
* Sentinel is annoying with a large breed because the Sentinel tabs only go up to 100lbs. Once your dog is over 100lbs, you need to buy two tablets and it becomes more expensive.
