That's a really tough question. I don't know your financial situation, so I wouldn't presume to tell you what amount of $ would be too much for you; however, I can share my story with you:
A few years ago, one of my cats was sick. I had about $750 worth of tests done (blood test, ultrasound), and the vet determined that he had lymphoma. I agreed to surgery to remove a tumor in his stomach, which was about $1500. After he came out of surgery, he went downhill. He wouldn't eat or drink and would shake (not sure if they were seizures or what). The vet said we could put in a feeding tube to help him eat. I asked what the prognosis was. She said that if we can get him to start eating again and start chemo, he would probably only live 4-6 months more. At that point, I made the decision to put him down because I didn't want to put him through anymore and could see that he was in poor shape.
In my case, the vet didn't tell me the prognosis wasn't good until after the surgery. Had I known that, I probably wouldn't have put him through the surgery in the first place. This was a few years back, and to this day, I feel guilty for putting him though the surgery.
I have another cat with hyperthyroidism, and I spend about $25 a month on pills for her plus about $100 every six months for vet visits/blood work. She is very old by cat standards (17 yrs old), but still has a good quality of life (eats, drinks, walks around, not suffering, etc.) so I don't mind spending that.
I'm sure you will make the right decision for your pet. I'm sorry you are going through this.
