Don't let a pet suffer.
Don't look at cost, look at quality of life and if an animal is in pain. If it is, then be a caring pet owner and allow the animal to go peacefully.
If the animal is not in pain but terminal, then only you can decide what is reasonable to spend. However, to let your family [if you have one] suffer financially for an animal to live a short time longer is something one needs to reflect on before spending an obscene amount of money.