Kennel cough is extremely contagious – my dog has contracted it from random neighborhood dogs just being around in front of the house. It’s very easy to treat – they squirt some yucky stuff up the nose. It’s also pretty cheap. And doesn’t happen that often.
If you keep up with the regular vaccinations (once a year, maybe $50 on average. YMMV.) and keep the pet(s) inside when you’re not around, you’ll probably go to the vet once a year. Especially with mutts. Purebreds tend to have more medical problems. The rule of thumb I’ve heard: take whatever you spent on your purebread and double it – that’ll end up being your annual medical costs. For an English Bulldog, pet insurance might make sense. For a random dogzilla, no way.
The first bill will be the highest – startup costs always are. If you get a puppy, I highly recommend crate training. Pricey startup, but will save you a fortune in not-chewed furniture, clothing and valuables. (My pets only destroy what I own outright. If I still owe money on it, they must know and leave the object alone.)