Dog has allergies I think, what will the vet do?

So my little stinker has allergies I think. First it was red bumps on his head then under his chin, then his ears got black stuff in them so I thought it was ear mites and he started scratching all over so I brought him to the vet, she tested him for mites and worms and everything came back negative so she gave me a steroid and antibiotic for his scratch marks. Everything cleared up and even tho he was neg for mange she gave me revolution to treat mange as well. If it all came back then she said it wasnt mange it was allergies. I am giving him nature balance dog food with allergy stuff in it and this is the food i switched him to. So I am thinking it is in fact allergies and before I bring him to the vet what can I do to keep the cost down, what should I have her do or not do? What will she want to do, and what should i be aware of? How costly will a dog with allergies be, from what I hear this can be very expensive..