If your dog is treated soon enough after being infected with heartworm then the dog will usually live out it's normal lifespan as long as the heartworms and heartworm larvae have been killed with the heartworm treatment.
It's also a good idea to have your dog tested again 6 months later after the first heartworm treatment to be sure all the larvae have been killed in the dogs system.
The heartworm if not treated soon enough could kill the dog within a few weeks to a month or two depending on how severe the heartworm infection is.
If your dog has become already infected with heartworms then the treatment cost for the heartworms from the vet is between $400.00 to $1,000.00 which would usually come out of pocket unless you have pet health insurance to cover the cost of the heartworm treatment.
If you treat the dog for heartworm at home to prevent them from getting heartworm then it averages out at a cost of $15.00 to $30.00 per month for the heartworm medicine.
However I would not recommend giving your dog heartworm medicine unless they absolutely need it because the heartworm medicine can cause other health issues and could shorten your dogs life.
A dog will usually get infected with heartworm after being bitten by mosquitoes so if your dog has been bitten by mosquitoes then it can get heartworm and then you should give the dog heartworm medicine or take the dog to see the vet.