Here is a link to Seth Resnick's Photo Pricing Guide. It does not deal with the budget market, but may give you some ideas.

Seth Resnick Photo Pricing Guide

www.editorialphoto.com might have useful info on the topic.
To get an idea of library prices you might check a price calculator on some of their sites such as Alamy.com
Be careful to choose a Licensed image though, because there is also the royalty free which have a different set of specs.

(I know this doesn't really belong here, but people whose opinions I trust and respect are in a small minority over at photo.net these days. Michael, please delete and accept my apologies if you don't want this sort of topic)

I've had a request from a book publisher to use some of my pictures of Indian landscapes. They say they're a low budget operation and I believe them - I can't see the Danish yoga book market producing too many multi-million bestsellers - but I don't want to give my work away.

I honestly have no idea what the going rate for low-end book publication would be - can anybody enlighten me?

Check out the Authors Guild.  You might also consider joining.  They fight for the rights of the authors who cannot dictate terms to publishers.