How much is too much??

Okay, so we all have pets that we love. I have a cat. I have had her since she was about 8 weeks old. Love her to pieces. But then I had my son. And as much as I love my cat, my son will always come first now. So now my cat is sick. She has lost 5.5 pounds. She is 12 1/2 years old. If she has what we think she has, then to fix it would cost me over 1K. So is that too much? When I don't have that much spare cash sitting around? How can I watch her waste away?