I'll pay as much as myself and my family are able to afford. Example, we spend probably close to 1,000 dollars in vet bills on my rooster Skoggi. However, those bills kept him alive for 5 years, instead of the 1 year he would have had otherwise. It was well worth it to me and everyone involved. I've pitched hundreds of dollars into stray cats, who now all have wonderful homes and happy, healthy lives. Having some of my adopters sending my pictures of their pets with stories about how well they are doing makes it all worth it.
However, there have been times when I simply did not have the money to treat and have had to euthanize. I feel it is more humane to do this than let the animal "die naturally," because when I was 9 my mother let my favorite rabbit die naturally, and it took months of his face rotting off, his legs becoming paralyzed, and being in excruciating pain before he finally died. I'm moving to Oregon so I can get euthanasia when this kind of crap happens to me; I've been chronically ill all my life and I'll be damned if I'm going to cost my loved ones money so I can be a doped up veggie on a resperator.