oh my.. Welcome to the site.. I don't have any experience with mites but do have experience spending tons of cash to figure out skin issues. Where are you from? Would you consider another vet? also @JAKEISGREAT always refers people to a plan .. forget the name for financing these bills.
I feel terrible for you. Duke had very bad skin issues. We finally gave up on the reg vet and took him to a dermatolgoist and ever since we got to the bottom and he has a regimin now that keeps him scratch free.
I know others will chime in.. hang in there..