Ok I think people are taking this wrong... Of course I will put my pet down if and when it becomes necessary. BUT, no, I will not let anything stand in the way if I or the vet feels that whatever the problem is, is curable/fixable... If your dog say, gets hit by a car and doc says ok it'll be $2800 possibly more for the surgery, you're really just going to say ahh screw it, he/she had a few good years? Not me, that's all I'm saying. Also, if people only post on threads to create conflict, you better expect a little heat back!!
The question how much it costs should not be what guides this decision. The question should be is it fair to the dog, will he be able to do doggy things again. Dogs heal up amazingly quick after trauma and other surgery, if there is a good chance of recovery, I would gladly spend whatever is necessary in trauma and orthopedic care.
Transplant is a bit of a different issue. First of all, there is a 'donor cat' that didn't exactly sign up for this. If we believe that pets are more than things, we have to look at this under an ethics perspective. In human live-donor transplants, donor gets something out of it, usually the knowledge of having helped a loved one, relative or friend. What does the donor cat get out of it ? Even for the recipient cat: Preop time on dialysis, 2-3 weeks recovery from surgery, ongoing cyclosporine treatment with blood-draws, still a sizeable risk of death, risk of developing cancer from the immunosuppression etc. Have we ever asked the cat whether she wants to go through with this ?
Maybe I am just not a cat person.