Sure cost per b/w print on color machine will be more expensive. Figures - entire machine is running anyway - Transfer belt / fuser are working, CMY developers and drums are turning and wear somewhat anyway, generally, only lasers are not drawing image and CMY toner is not being consumed. My experience shows that issues with developer units in big part comes from amount of rotations, drums wear from friction against cleaning blade and primary charge rollers pick-up dust and debris regardless if any of 4 printing units laying image or not.
I sell service contracts for b/w copiers $0.08 to $0.01 per click in my Office Equipment Sales/Service business but We had decided to put our Xerox WC7335 used in printing shop under contract with xerox (I am not too trilled with servicing color machines - our clientele mostly medium to large b/w) so they charged me 2c per b/w and 12c for color any size. We were printing max size, multiple ups making reasonable profit.
I would strongly advice you to get a b/w machine to run b/w jobs. As an example - we bought a Ricoh MP9000 for our printing division with 800,000 pages on it for $4500, spent another 500 on some parts. In less than one month we put nearly million more on it. This machine is nearly $30000 - 35000 new, I think, but regardless new or used there is no way Color machine printing b/w images will give you speed, longevity and value as b/w machine.
Right tool for the right job...
Same thing when people asking what is better - Digital Toner vs Indigo vs Di press - all these machines have their own niche and perform best when used where they meant to be.
My 2c
