Sorry to hear that. Such a difficult decision. I take it you don't have insurance?
We do have insurance, but I stupidly got a policy with a £2k per year claim limit as I figured a housecat would be ok with this. We have already spent £2.9k on blood tests/ ultrasound / X-Rays to try and diagnose the problem as it's really hard to diagnose (Dry FIP) so that is the insurance tapped out. There is a treatment you can do through the vets which is essentially Remdesivir but its twice the cost of the alternative we have found.
We have had some progress though - the vet suggested that we look at a page on Facebook - FIP Warriors which is a whole community of people in the same boat. There is a black market treatment that is having amazing success rates (95% roughly) and whilst still expensive it will be in manageable chunks over the next 12 weeks. We started injecting him this morning after I went and picked up some of the supplies and vials last night and fingers crossed by the end of Feb he should be cured. It's going to be difficult though as its daily injections, which apparently sting a bit due to their acidity. He was not a happy cat this morning and as soon as he susses the routine he will probably start hiding!
I was really sceptical about using a non vet-based treatment at first but after meeting someone in person (and their cured cat) it was an easy decision to make as if we do nothing he would 100% be gone. Hopefully nobody's cat here will ever have this issue but if you do please feel free to reach out as I am more than happy to help.
Signs to look out for are very vague and could relate to other things, but its usually in cats under 3, pure breds are more susceptible. Frank's coat got into a really bad state, he had stopped grooming properly and lost about half a kilo from a reduced appetite so we took him in to the vets. It was then when they did the blood tests and noticed his Amulin/Globulin ratio was low at 0.4 so they did more tests. It can onset really quickly so you have to act immediately.