My little man loves going outside so I don't have that cost, and he's fully trained to go when we go for our daily walks. It's cheaper and the exercise wouldn't hurt your dogs. How about training them on one of those indoor toilets. It will cost a couple of hundred at first but they go on it and it's washable and flushable.
Puppy Potty :: Dog Potty Training :: Indoor Puppy Toilet :: Puppy House Training & Housebreaking Potty from Eco Pet Solutions, Inc.