All standard publishing contracts are enormously lopsided in favor of the publisher. In textbook contracts there is no such thing as standard royalty provisions. Having said that, a review of more than 100 TAA author contracts in my files does reveal some common "ranges."
"Standard" ranges. The concept of "standard" royalties is less common in textbooks than trade books -- so much so that it is almost counterproductive to state ranges. I am concerned that young authors may put too much stock in a so-called standard range, while more seasoned authors may find it contrary to their experience. Accordingly, view the following as indicators only, and don't be afraid to push for royalties appropriate to your stature, leverage and revenue generation for your publisher -- even outside the ranges discussed here.