Paying my mother to babysit

I'm 18wks pregnant with my first baby. My mother suggested that she would retire early to help take care of the baby. I told her that choice was up to her if she could afford it. At no point did I ask her to do this. She also asked if my husband and I would be able to pay her. So my question is should I pay her and if so how much? We live next door to each other so there is no travel involved. Everything the baby needs will be supplied, she won't have to purchase anything for the baby at all. I work 10-15 min away from my job so I am very close to home and there will be no late evenings or early mornings that she will have to do. I leave home by 8:45am and am home by 5:30pm at the latest. Any suggestions?!