You should be careful who you ask about the cost of veterinary care for your dog. Even a phone call to a veterinarian's office will likely get you the answer of "There are so many factors involved in the treatment of heartworms, that one specific answer to your question isn't possible."
Heartworms cause a lot of damage to a dog, not only to its heart, but also to the lungs and other organs, depending on the severity of the infection and any other medical problems that are present. For that reason, your veterinarian would be doing some simple lab tests to determine what other considerations demand attention. That process would influence the treatment price before the actual heartworm infection would even begin. Then, there a couple of ways to treat a heartworm infection and, depending which of those your dog would require, the price would be variable.
Again, web sites won't help you with your question. In fact, most of them will only confuse you even further. You would most benefit from a visit with your veterinarian to discuss your very questions about the overall cost for the treatment.
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