First things first
Welcome to Yorkie Talk!
I am so very sorry that your little one is having such terrible skin problems. I am not a vet nor have I had serious skin issues, but it seems like you might be right about your vets taking you to the cleaners. Have you considered looking for a different vet?
Has the vet done any skin swabs, cultures, or used a black light to rule out a fungal issue? If so, I would strongly recommend to start from the inside out rather than just toy around with shampoo/conditioners. Look at the whole dog, not just the skin -- what is he eating, where does he sleep, where does he spend his time.... maybe an environmental allergy is the trigger for an allergic reaction.
Good luck! Please keep us posted.....