It depends.

If the chances of recovery were slim, and/or the cost completely prohibitive, I would euthanize the dog. For example, if the vet said, "Your dog has cancer. The treatment will cost $10,000 and there is less than a 50% chance he will make it", I would euthanize him.

Over the course of my adult life, though, I have spent untold thousands of dollars on vet care for animals.