I work in a shelter and during my time there I come across cats that get diagnosed with FIP every now and then. Until 3 years ago there was no treatment and we always had to let them go.
Now there is a possibility to treat them – however where I live (Germany) veterinerians can not give you the medication, as it is not avaiable for animals.
You have to buy and administer it yourself. There are several groups you can find online that help you with the treatment, depending on where you live.

However you still have to go to your vet for diagnosis of course and you need regular bloodworks during treatment, and also to make sure there are no other problems arising.

Our shelter is lucky to have their own vets with a clinic, so we can do a lot ourselves.
Everything I do and I mention here is always in agreement with our vets.

We made the best experience with a treatment of daily injections (though there are pills avaiable now), given at the same time for 84 days. Dosage depends on brand of the medication, weight of the cat and form of FIP.

During this time we have bloodworks done at least 3 times:
1st for diagnose , 2nd at 42 days to see if everything is doing well, 3rd at 80 days to see if we can stop treatment. After 3 more months after the end of treatment we do another bloodwork to see if everything is stable.

So far I treated:
Elli – female – FIV/FelV negative – 3 months when diagnosed – ocular FIP (had neurological symtomes at the beginning) – duration: 84 days
Tigger – male – 1 1/2 years when diagnosed – FIV/FelV negative – wet FIP – duration: 84 days
Miraculix – male – 8+ years when diagnosed – FIV/FelV negative – wet FIP – duration: 84 days
Erik – male – 1 1/2 years when diagnosed- FIV positive/FelV negative – wet FIP – duration 98 days – retained increased liver enzymes, otherwise healthy
Flynn – male – 1 1/2 years when diagnosed – FIV positive/FelV negative – wet FIP – ongoing

Elli was born at the shelter, all others were found outside. All except Miraculix developed FIP when they were already vaccinated and neutered and ready to move out.
All recovered fine without further problems. Erics liver enzyme rised during treatment, which can be normal and usually goes back after treatment stopped. However he has still increased livernezymes, despite the treatment being done for overe alf a year. But he is otherwise perfectly fine, so it was worth it.

Symptoms were nearly the same for all: fever, apathy, not wanting to eat and for wet form fluids in their belly.

I accompanied more treatments privately and if the treatment started early enough all recovered.

In our shelter two cats didn’t make it, however both never were diagnosed for FIP, as they died before the bloodwork we have to send away was finished. One was found and brought to us already weak and unresponsive, bodytemperature of only 35C. She had the typical fluid in her belly, so we started the treatment without waiting to hear back from the laboratory. We tried to get her stable, but she was already too weak.

The other was around 5 month old and also positive for FelV, so at first it was suspected that’s where the trouble came from. His liver already made problems, he had high fever and refused to eat. He got treated with antibiotics, fluids and more. When he developed fluid in his belly, we started the treatment, as he was growing weak fast… he died before we got the message, that he had FIP, too.

The costs consist of (rough estimates, can vary widely depending on where you live)
Medication: ~1000-1500€ / 1100-1630$
additional things like syringes, needles, vitamin b complex, liver tablets…: ~100€ / 110$
Vet Costs: ~600-800€ / 650-870$

Despite the treatment being very expensive, it is definitly worth it. If started in time and no other accompaniying problems are there, all diagnosed and treated cats survived.