You'd pay the cost for each lens! Good polarizers are expensive.
Actually, I'd like to see someone build one into the camera. Using a button the camera would slip the polarizer in and out. You'd need an efficient interface to rotate it, like press the polarizer button and use the control-dial or a fly-by-wire ring.
It would be really neat because you'd only pay the cost of the polarizer once and it would be for a small one since it would be at the back of the lens. It would save time too. I find screwing on filters rather slow (Why don't they make bayonet lens-filter connections? It made sense for lenses and hoods and so much faster).