He's the best cat in the world...

My cat Po (Popoki) has become really sick. I brought him to the vet last Thursday night, and blood tests came back showing that he had anemia. What they didn't show was why? Po went into the hospital last Friday to get an ultrasound of his lower abdomen, to check for bleeding, masses...and whatnot. There was swelling in his stomach and his intestines...but more blood work didn't explain why. It's been thought that he has pancreatitis...and we're waiting on more lab work to come back hopefully by tomorrow. Po came home last night, because he hadn't eaten since last Wednesday, and we wanted to see if it was just because he was really scared at the hospital. He did eat, but I brought him back today because he was dehydrated and felt that they could provide better care. Now they're talking exploratory surgery to do biopsies for cancer or some other medical problem. Estimate for open surgery is $3700 and they can't guarantee anything. I have pet insurance...but they can't tell me what they'll reimburse...until they have a final diagnoses. I'm about $2500 into this already. I always told others that pet insurance would keep you from having to make a decision based on whether or not you have the finances to provide your pet the medical care they need...but that's exactly where I'm finding myself now. And I don't know what to do...