Digital BW, The Print

At those prices, if you were going to make several different books 
(or use them often) it might be worth getting a CMYK profile made for 
their printer. Then you convert all you images to that profile, and 
they send the job through with their normal no color management 
workflow. The target might make an interesting liner page before 
getting to the index (etc.) or maybe all the way at the end of the 
book. Using a profile this way should allow you to use only the black 
ink, so as long as the resolution of the printer was high enough, you 
should get good output.