Then you need to get a prescription from your vet and go to Drs. Foster and Smith. Even if your dogs are 70 pounds, you can save $20 for a six month supply over what you're spending now.
At Petmeds.com it is about $80 and at our vet, it tends to run about $10 more. Of course, that is for two dogs and is for a 6 month supply.
Someone told me that you could actually get the pills for the biggest of dogs, break them in half and save that way. Have never done this though.