my baby had fluid around her lungs

I went away and when I got back last week she started to vomit everytime she ate anything, and went from small to almost nothing in days, and developed heavy breathing, which I assumed was a result of being so tiny now.........
I took her to the vets and the found she'd got fluid around her lungs, he took away some and did xrays but couldn't be definitive of cause so far, so he gave her antibiotics and a course of diurectics, and prognosis is that if she responds to that then it could be something they could treat if not then
he thinks its most likely a tumour or some cat disease (cant remember what) but she never goes out so.........
but there was still not enough proof without further investigation, which will be very very expensive and most likely not going to have an outcome that can be fixed......... I don't really no what to do at this point, any advise is really welcome x