Certainly not a correction, (I think bleach will always be cheaper) but I saw a product at the Orlando Pool Show that intrigued me. It was a SWG that was made for spas. Pretty close to brand new, I think.
It simply plugged into a 110v source (thru a transformer that reduced to 12v) and then was draped over the edge of the spa into the water and it began generating chlorine. It was around $250-275.
It had no moving parts (you "tapped" it to change percentage output) so it was well sealed. If you removed it from the spa, it shut off.
The only thing I questioned was by simply draping it over the wall, I wasn't sure if the concentrated chlorine production that close to the fiberglass wall would have any negative effect.