The lesson of your sad experience might be to question the vet about likely outcomes before such expensive tests are run.
That's Urchina's point, though.... there are many situations in which the possible outcomes may differ drastically, depending on the diagnosis. And the diagnosis can't be known until tests are run.
To present a somewhat-common scenario that I see as a veterinarian: Imagine that you take your dog into the emergency clinic for sudden weakness/collapse that occurs in the middle of the night. The veterinarian runs some very basic/preliminary tests (physical exam, bloodwork & abdominal xrays).... and now, after $600-800 in testing, advises you that they suspect a mass on the spleen and blood in your dog's abdomen. There are three possible causes for this - one is a very malignant type of cancer called hemangiosarcoma, one is a benign tumor called a hemangioma, the other is a hematoma which is basically a blood blister. Surgery to remove the spleen is curative for a hematoma or hemangioma, but the prognosis is poor for a hemangiosarcoma. If you decide to go ahead with surgery to remove the spleen and submit samples for histopathology, just to find out the diagnosis and your dog's prognosis, you could easily be out another $2-3k or more... your dog may first need a blood transfusion and hospitalization to be stabilized for surgery and then there's the fee for the surgery itself. Then additional costs of post-operative care, histopathology, etc. When it's all said and done, you may spend $3-4k+ and have a completely healthy dog that goes on to live a healthy, normal life... or you may have spent $3-4k+ for a dog with a very small amount of time left. There's no way to know without doing the surgery & histopathology.
Obviously, every owner is going to have a different opinion of whether to go ahead with surgery in a case like that... and that's completely okay. I practice in a low-income area, so probably less than 20% of my clients would go ahead with surgery in that scenario, even in the case of a young, otherwise-healthy dog, and I understand and respect that. They just can't afford the surgery. The point, though, is to be prepared and to understand that you won't always know that you're spending money on a "bad disease" until after the money has already been spent. Vet care was cheap in "the good old days," because vets would palpate a splenic mass and a belly full of fluid and say "it's probably cancer - we should put her down." Now, we've learned that there are other options and that some scary-looking things aren't bad and can be treated effectively.... but clients need to decide to what extent they would want to take advantage of those options and sometimes that can be challenging.
Just this last week, I had a client bring a cat in with the complaint of "blood in urine." This is a cat that supposedly was previously healthy, but had never had any veterinary care. When I opened the carrier, the cat looked like crap. Yes, large amounts of bloody urine were leaking from the cat, but the cat was also very weak and dehydrated. I recommended bloodwork (including feline leukemia/AIDS test) and a urinalysis to start, with supportive care (IV fluids, etc) likely recommended after that... the initial estimate was something like $250 just for the minimal diagnostics, not even including any treatment. The owner couldn't do it... all that she would authorize was the urinalysis and then antibiotics for the cat, because she was convinced it was just a UTI. I repeatedly told her that I was concerned about other bad things, prognosis was guarded without appropriate treatment, etc.... and yet then she called and was angry at ME when the cat developed seizures the following day, saying that it was my fault because I didn't warn her that could happen. I get that she didn't have money to spend on diagnostics, but she didn't get that I couldn't treat her cat without a diagnosis. I wish that I could have offered a more educated guess regarding the cat's underlying disease, but unfortunately I can't do that without any testing... and that is often the case. Last I heard, she was going to take the cat to the animal shelter for euthanasia... and that was probably her best option, if she had no money. But the cat may have had something completely treatable.... there's no way to know without testing.
Urchina - so sorry to hear about your loss. I had one of my own euthanized last week and it sucks. Your dog was lucky to have a family that cared enough to give her a chance.