One of the pleasures of being a hobby farmer is that you can spend as much as you like on your stock without having to look at the bank balance, and care about individual animals as individuals, thank goodness.
Does she seem in pain? Could it be some form of mechanical lameness? Possibly daft, but I had a pony with a swollen (scar tissue) tendon sheath that caused him to appear lame, but the vet said he was in no pain, just restricted in his movement. If painkillers don't help, then maybe pain isn't the cause of the lameness. Is she carrying the leg, or just nodding? Could it stem from her back rather than a leg?
Maybe she's worked out she gets extra fuss if she hobbles...
I assume you've ruled out footrot etc, but are her hooves in good shape?