This is tricky. To be honest even a base figure I'd probably ask minimum £100 per page or spread of image. (depending on the brief and complexity). You'd need to build a 20% contingency into the total - in case of excessive changes, new pages, etc.
Then I'd charge completely separately for layout design, as it's a different skill. My standard charge for that is £15/hour for artists and that is dirt cheap. If you already have a graphic design rate from previous work (which it sounds like you might), then go with that rate or a special reduced version of it.
If I were going to do book design at a flat fee I'd put it at £200-300 for design only and final printer file - additional hourly for updates and edits - and again you need to take into account changes etc. Also keep in mind that if you are doing the layouts and dummies you're likely to be the only one who can edit her project, so you will also be the editor during the whole project.
There you go. Convert to dollars and see if that helps you at all.
I've learned the hard way over the years that if THEY don't have the budget, that is not my concern. I'm not a charity either as an artist or a print designer. (remember that if you aren't paid well for your artwork, a self published book is unlikely to get you any further recognition other than a resume line, so analyse what you're really getting out of it)