Royalties and advances from academic publishers: what should I look out for?
For academic books, there may often be no royalties.  And only a small number of copies sold.  Maybe if your first book has unexpectedly large sales, you could negotiate something more on future books.  

Typically, though, academics publish books not to make money from the book, but to build their academic reputation.  And that may lead to promotions, tenure, hiring at more important universities, etc.