There is no easy answer for this. For most of us who choose to have pets, I believe we view them as family members. However, there are of course people who get pets as "accessories" or "interactive toys".... but I think most people who share their lives with animals in their homes or barns view them as family members.
For us, it's about quality of life, vs. finances, vs. quantity of life. (Quantity means length of time lived, quality means in good health/happy vs. unhappy/sick/in pain). I cannot in good conscience spend the same amount feeding my dogs as I do feeding my family. That doesn't mean that I haven't obsessively researched dog foods to find the right one that is least expensive and is high quality, for our beef & chicken allergic GSD cross (so we go with lamb & rice, and avoid beet pulp and corn as fillers because they also cause a reaction). I could not condone spending $2 or $3 just for a 10% chance of survival, but often, it is not that clear cut--often it is so much of a grayer area.....
We had a tumor removed from our GSD cross. She was given a guarded prognosis (the cancer will likely come back and metasize; there is no timeline, however). We were given options for treatments (chemo, radiation, and both are expensive), which would extend her life, but wouldn't improve her quality of life. In the end, we opted not to do either. She's 11, and has lived a good life. There is no sign of visible recurrence as of right now (and only monthly, very expensive ultrasounds 'might' show if she developed internal tumors; the original tumor was on the skin). If she has any more tumors that we find on her skin, we will have them removed (its $300-$500 per tumor) per the vet's recommendation. If she becomes uncomfortable but is still willing to keep going, we will work with the vets to help her get comfortable. And when it gets too much for her, we will send her peacefully on her way. I dread that day. But I feel that my job, by choosing to have her in our lives is to provide her a good life and decent vet care, balanced against the needs of the rest of our family (who will probably outlive her by many decades).