Books that have many images seem to require subventions before they can be published, but how about maps? If maps can be rendered in a manner so that they can be printed on the same page in the same ink as used by the text, does this increase the cost to publish the book?
Maps will generally incur the same production costs for printing as any other images. Often, printing images in color, either in a photo insert or (at greater cost) throughout the book, will require a subvention. (Here is a good explanation of book subventions.) In the absence of a subvention, if maps and other images can be reproduced in grayscale that will indeed bring costs down. One consideration is whether there is any important color information in the map that will no longer be apparent when the map is rendered in grayscale. It’s a good idea to ask your contact at the Press about workarounds for these situations.
It’s worth noting that altering a map so that it is in grayscale and not in full color would not have any effect on the licensing requirements, if any, for the map. Licensing fees and permissions for maps and other images are usually the responsibility of the author (refer to the terms of your publishing agreement).