Should i rsvp as yes or no?

okay, so my friend is having a bat mitzvah. she needs me to rsvp by like TODAY. the problem is, my mom won't let me go, as of now. im 70% sure she's going to let me go if i talk her into it over like a week, and i cleaned my room and did the laundry today, im preeetttty sure i can bribe her into it. so im thinking im going to rsvp yes, but if i can't i'll have to cancel on her and she'll have spent money on food and goodies and other stuff. but if i rsvp as no, i can't go at all, and i REALLY want to go because its like THE most extravagant bat mitzvah ever. what should i do?