I was thinking about doing this and did a google, I wouldn't be saving anything! My vet charges $72 for 12, and I got a 12 dollar rebate, which lowered the price to $60.00 no tax no shipping. By the way, Heartgard will pay for treatment, if your dog gets heartworms, as long as you have proof of purchase, so save those receipts!
Prices quoted below are for 6.