My cat is vomotting brown-green liquid several times a day... the 2nd day he was sick and very lethargic I?

took him to he vet. He wouldn't drink or eat, extremely lethargic, still throwing up. Of course he was dehydrated so they did a urine test, blood test, feline leuk., felt his stomach which wasn't tender etc. They are keeping him for 3 nights giving him electrolytes for dehydration and anti-vomiting meds and IV antibiotics. Still throwing up after all this, tried antibiotics by mouth and he immediately threw up. They call me today and say they don't think he's diabetic they'll do a abdomen scan and more blood and urine tests. I'm up to a $850 bill already and they still dont even know what's wrong or how to fix him?!