bluenose2 - Does your aunt have a regular vet that she calls for health related matters? If so, I would start with him/her. I know that our vet offers in-home euthanasia for clients. I am not surprised at the cost of $500.
The only other thing I can think of is if you or another family member takes the kitty into the vet practice. The cost will be high but not nearly as high as $500.