There is health insurance for pets. I have never bought it, although some people tell me they're very glad they have. I do have a monthly envelope for veterinary fees, just as I have for grocery money, gas money, and other household expenses. Most of the time, that money just piles up, and then something happens with one of the four-footies and I'm glad the funds are there. It's the only way I've been able to pay for some of the things I've had to pay for.
You might ask your vet about brushing your dog's teeth at home. It won't entirely replace vet cleaning but it sure helps. My dogs have informed me that doggy toothpaste is yummy, but they let me stick the long toothbrush in their mouths even though they'd prefer to lick the stuff right off the brush.
The heartworm treatment is essential, of course. We do the medicine but don't have to have the test. We also haven't had to worry about fleas and ticks where we live (unless we go to the mountains), but suddenly there's more concern about lice (yikes!) so our dog is now getting Frontline for that. Sometimes you can get these things less expensively online, but that's something else to talk to your vet about.
It's not easy, though. We've had the same vet for years and years, and when the children were young I sometimes had to call the vet office to say I knew I still owed them money but couldn't pay them until next month because this month all the kids needed shoes. The advantage of finding a good local vet and staying with that person is that when people really know you they will trust you!
But then, I'm not a practical person. With the exception of the CCI puppies we raise, most of our pets have come to us (rather than being purchased), and I take them in and do the best I can for them. I do as much as I can at home, and am extremely glad my vet doesn't charge for phone consultations! Happily, the vet has helped me say no to some costly things that all the advertisements say pets can't live without.
By the way, I hear you about the Fancy Feast! One of our kitties developed a major thyroid problem. He had medicine (with a monthly cost of, probably, a college textbook) twice a day, and in addition to the regular dry food I gave him Fancy Feast just to put calories in him. The the other cats said, "How come HE gets the special stuff?" so they got a teaspoonful when he got half a can. He died (of old age) last year, but the other cats still want their teaspoonful!