I just spent 691 dollars at the vet. My two dogs needed boosters for their shots. We have a lyme disease problem here so they got that. Then there is rabies (lots of bats have rabies around here). Then there was distemper, which I bailed on for one dog as it is a dog-dog transmitted disease and getting 3 shots at once can have some bad side effects. We'll get that one later.
This all cost about 270 bucks. I then bought enough heartworm preventative medicine (interceptor) to last 2 years for both dogs. 320 bucks (or so) for this. Ow!!!!
According to this site in my area (north east US) there is a very low incidence rate of heartworm, between 1 and 5 cases per clinic per year.
Here is a pretty comprehensive description of heartworms, symptoms and treatment. The little worms are delivered into your dog by mosquito (damn things). But the symptoms are visible and treatment costs around 300 dollars.
Why the hell would you spend 80 bucks per year per dog to prevent heartworm? First off, in the northeast, where we have lyme disease, we use advantix (unless you also have cats!), that keeps off ticks, fleas AND mosquitos!
While I wont tell you what you should be doing... I'm going to return my 320 dollars worth of heartworm medicine.