Vet costs for Duster

For the past few months my cat Duster has been sick. Duster is 12 years old and i have raised her since the day she was born. We have been through so much together, she's been my rock, my reason. She always knows when I'm upset and anxious and come to comfort me.

Then it happened, she was getting black wax in her ears plus her anal glands we swollen.. Of to the vet. They said she didn't have mites or infections. And they dewormed her. 
She continued to get worse, still black waxy ears, cat acne on her chin. 
We were given revolution. Things got worse even more so. She started getting black patches on her back feet. Losing more fur. We got a rinse for her ears and a week of Baytril.
And still it continues to get worse. I discover all the fur on her back legs and tummy were falling out. Her tummy is red and raw.
Another trip to the Vets, their tests don't show anything, but they sent 2 samples away. 
And was given an antibiotic shot. 
But she is still not any better. She's losing more and more hair.
We are waiting on her test results. If they show nothing she will need a biopsy and blood work done. 
I have spent nearly $1000 now on bills with potentially more tests on the way.
Any help is greatly appreciated ❤️