I had to have my first snake put down due to viral pneumonia that turned septic. It cost about $250 to have her injected first with anesthesia to knock her out so she didn't feel anything, then the euthanasia drug. Call me sentimental, I had her cremated and still have the ashes in an urn on my mantle over the fire place.

I think, if you're going to cull an adult, it should be done by a vet as painlessly as possible. I thought the anesthesia necessary given the way they have to put smaller snakes down. It's quite painful.