What to do if i lost a textbook and my mom won`t pay for it?

Hi. About 2-3 weeks ago. I lost a textbook and have been looking EVERYWHERE for it. I checked the hall floors, with the janitor, the library and all my classrooms and home. I told my mom she needs to pay for a replacement one which is 85.00$. She won`t pay for it and says she is going to talk to the office or my social teacher. I also told her if they find my textbook they will refund the money back to her and yet she still says no. i don`t know what to do now since she won`t pay for it and my final exam for social is monday and that is when we need to hand them in. :( Please help???