One of my girls get's sores that weep and
scab over, that she scratches to death, and
it's attributed to seasonal allergies.
My vet recommended I try Benedryl, and I've not looked back. I start in September, and she takes it through November, then again in March through May (those are all my girl's allergy seasons).
Check with your vet on the dose, but it's gotta be worth a shot, right? Surely it's cheaper than all the other stuff you've been trying.