While I am not a vet, it isn't natural for an animal to not eat and drink daily. I think you would want kitty to be healthy no matter what. I would get surgery for him/her, and hope for the best. I also think you might want to have it checked out by another vet, as this one seems to have been a little off with finding something out for you. Hope this helps, good luck.

 The prognosis and cost depend upon the diagnosis.  Fatty liver disease is secondary to not eating and can be treated with liver protectants and supportive care--sometimes they need a feeding tube.  Certain types of cancer are treatable in pets and the prognosis is variable.
The next step would be to do an ultrasound guided biopsy of the liver to try to pin down a diagnosis.  Abdominal exploratory surgery can be done too, this allows us to examine all the organs and take larger biopsy specimens.
Costs in veterinary medicine vary widely between clinics and areas of the country.  Talk to your veterinarian about the treatment plan and costs involved.