Yeah, deformaties happen quite often, but they are hardly worth more money to pay for them. Many don't live a full life, but some do, and if you wan't to give it a good life then great. It's not something i'd look at and go- OMG cull it.
I don't understand the price they request for a deformed ray. I personally wouldn't kill it. I'd feed it like all the rest of the rays and give it a good home. I have one ray that has half of a back fin and clasper from an apparent attack when he was younger. He was skinny as heck and had a skin issue when I rescued him, but he deserves a shot at life. Now he is fat and happy in my tank, and heck he may get some action with the one clasper he has left!
If it;s birth defect, then fine treat it like a normal fish. Price should be discounted, not raised. If it's cut, the person that did that should be smited off the planet.