Is there a way to save my students money on a textbook?
An easy thing to do that can be very helpful to your students is to put a copy of the textbook (or two or three copies) on reserve in the university library.  Students can then photocopy critical sections of the book (e.g. the homework exercises.)  This is particularly helpful at the start of the semester when students are waiting for copies of the book that they've ordered online to arrive.  

If you have control over the choice of the textbook, you should consider moving to a cheaper book or even an open educational resource (OER) that is completely free to students.