OK, I understand the importance of a bat mitzvah, but I didn't know that it included formal photography. Please pardon my Catholic upbringing. I have a good friend who is Jewish and I usually call her when I have a question about her faith. If she doesn't know, she goes to her son, whom she calls Rabbi Max, for the answer.
Anyway, my experience with formal photography comes from my wedding. Our deal with the photographer was to receive our formal album, and two parents albums, plus we got a proof album. Let me tell you, although we love our formal album, I really like the proof album because it tells the story of the entire day, not just snip-its from it.
Since I don't believe I'm qualified to tell you that you should or shouldn't also do a formal album for a bat mitzvah, I will tell you that I am glad that I have my formal album, but I really love my proofs. Only you will know how you will feel when the day is past.
Do you have the option of making that decision later, once you see the proofs. At that point, you will really know what you want to do instead of speculating now about it.
OTOH, you can use our mode of thinking about some things that are expensive.... "oh well, it's only a couple more home equity payments.." lol