I agree with Joanna - priorities of the whole and quality of life are the determining factors.

Myself and my BF come first in the $$ scheme of things (food, rent, bills...), but I do everything in my power, financially and physically, to give my animals the best life I can give them. I have a savings account especially for the animals, mostly for future vet bill coverage. Of course, I cut corners wherever I can - hides, bedding, decor...but NEVER on essential things like quality food or reliable equipment.

I've had to put pets down before, not only because surgical procedures were beyond means but because the animal might still have suffered afterward. More recently, when I took the little pastel in to the vet I did so prepared to pay for the service, but also ready to have him put down if it looked hopeless. Now, he's the prettiest and plumpest male in the group.

When faced with extraordinary vet circumstances, i.e. things beyond normal maintenance, I always ask these questions:
How special is this animal to myself or my partner (emotional ties, therapy pet)?
How much extra money can we give toward its care?
How much MORE are we willing to scrape together, if needed, to keep the animal healthy?
What will its quality of life be with the treatment and without?

I rarely ask how much the animal is worth...since even when I start breeding, these snakes will still primarily be my pets.