they show up on ebay from time to time, sometimes under $100, but it can vary a lot. They are a nicely made piece, adjustable diopter, bubble level (very handy with this lens), etc. Granted, not worth what MAC wants for them new, but easily worth $100 I'd say, all things considered and taking into account that the Mamiya 7 is on the higher-end of medium format systems.
Good info, I've only seen them at the $300 mark, I would most likely pay $100 for one, thanks. Next time, if I DO get a 50mm I'll be sure to buy a lens WITH the darn viewfinder... can't believe I didn't realize this when I got this lens.