I'm so sorry about your little girl.
The good news is that liver disease isn't a death sentence and you don't need to spend $5000 to diagnose it.
When the bile acid test was done, was your girl eating a high protein food or had she taken any drug (including heartworm and flea med) in the last week or two? These can skew the results. Because of the neurological symptoms, it does sound like a shunt or MVD (small shunting throughout the liver).
If she requires surgery, the best place to have it done is University of Tennessee and it is less than $1700 usually. A Protein C test might come next and maybe a scintography. Ultraounds can be inconclusive, but they can be helpful if bladder stones secondary to liver disease is a concern. Liver biopsy can be used to diagnose MVD but if shunt is ruled out, you can just consider it MVD, skip the biopsy and put her on supplements.
I think scintography is usually about $600.
If a vet wants $5000 just to check for a shunt without repairing it, I think I'd go somewhere else.
What food have they put her on and have you started supplements?
My dog's bile acids were post 106 at last check.
I decided on ultrasound because of some bladder issues (which again could be inconclusive) and no shunt was seen. She had a biopsy during her spay and no MVD was found. So it is still a mystery with her. She doesn't have any neurological issues though.
Has a urinalysis or culture been done to look for crystals in the urine?
Has a blood panel been done to look at liver enzymes?