One of my dogs had two bouts of cancer. The first was bone cancer in his jaw. After the x-rays, MRI and surgery to remove half of his lower jaw we were into it for about $4K. The money was hard but there was no question of spending it for my family and my special buddy.
Second time was about 3 years later -- eye cancer. He lived another three great years. And I still miss him.
This is him about 6 months before we had to put him down (due to kidney failure):
One thing that shocked the heck out of me -- an acquaintence who was (I thought) a dog person told me after the first surgery that I was crazy to spend all that money when it would only cost $50 to put him down...