I take it that your vet has ruled out bacterial or fungal infection...

If the cream is 1% hydrocortisone, don't waste any more money on vet's bills: you can buy the same stuff in supermarkets, sold for insect bites and stings in people (ASDA £1.50). I'd be slightly concerned about your dog licking off a steroid cream... The cream should break the vicious cycle of the dog aggravating the soreness by licking, once it calms the irritation. Dermatitis is often triggered by some kind of irritant, but you'd sort of expect more than just one foot to be affected. I'm no vet. Just ruminating. Good luck!