Our (3?) year old golden retriever had cancer on his butt. We paid for surgery. Vet wanted to have a sample tested to know for certain the type of cancer. Aggressive form. Vet said, "there's a possibility that it could come back." A few months later, the dog was having a lot of trouble breathing. Took the dog to the vet; he agreed completely with our decision to have the dog put to sleep to end its suffering. (It would only drink if the water was brought to him; wanted to stay laying on his side.) No testing done; in the words of the vet: "I think you're making the right decision. Sure, you could spend a lot of money on testing, and perhaps another surgery that would prolong his life for another month, but you and I know what the diagnosis of those tests is going to be, and the testing and surgery would simply prolong his suffering."
And, I know that Alky isn't going to like me saying this, but dogs and cats are put to sleep every day at shelters. By the thousands. Electing not to pay for an expensive surgery, and instead adopting a new pet simply changes which animal dies. The healthy one survives, the sick one dies. The only difference is the emotional attachment. That's one huge reason why such choices are different than when making those choices with humans. Or to put it on human terms, imagine that every day, they were euthanizing 8 year old kids. A 70 year old, unrelated person who had lived with you for 20 years had cancer. The surgery was going to cost $10,000. You had a choice: pay $10k for the surgery and extend the life of the old person by a few years, or pay little for hospice care, and then adopt an 8 year old who would otherwise have been euthanized. Plus, you'd have a shitload of money left over to possibly do some good for other animals (donate some to the SPCA or something.)