Laser printing in color is about 4 or 5 times more expensive than printing in mono. Not all our printing software lets us specify whether to print in color or not, so for large documents where we're not interested in the color appearance, we convert them to mono (or grayscale to be precise) using Ghostscript (which is free software, available for many platforms):

gs -sOutputFile=output-filename -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 input-filename < /dev/null

Because you run this on the command-line, you can easily use it in scripts for automatic or mass conversions.

Source

http://newsgroups.derkeiler.com/Archive/Comp/comp.text.pdf/2009-06/msg00046.html