Well, any pet is expensive if it gets sick! But yeah, it's a surprise when it happens.
I sure hope the antibiotics will get Xana well.
One thing, it sounds like the vet you found might not be an avian expert. For the future, you should do some research and find out if there's an avian vet in your area. With luck, you'll never need one, but it's a good idea to know.
If your keet has no parasites and no obvious injury or abdominal lumps or hard spots, all of which I assume your vet would have looked for, then I would guess the antibiotic treatment has a good chance.
And I'm very glad to hear she ate a little! Protein will help her heal from whatever it is she's sick with.
(Just make sure any "wet" food you give her, like egg or fruit, doesn't sit out too long, or it will grow bacteria and maybe make her worse.)