The widespread custom is to give 18 dollars or whatever the local currency is, or a multiple of 18. In Hebrew each letter has a numerical value — aleph is 1, beit is 2, etc. — and the word for life in Hebrew (Chai) has the numerical value of 18. The idea of giving 18 is to symbolize the importance and celebration of life. A “new life” of adulthood of Torah rights and responsibilities begins for a young lady at this age.*
I once heard an explanation of why “life” is connected to the number 18 in Judaism. We are taught in “Ethics of our Fathers” (Ch. 5) that “18 is the age of marriage”. Of course this is not an ironclad rule, and the advisable age of marriage nowadays depends on the customs of the place and the nature of the individual. Nevertheless, in an “ideal world” it appears to be the ideal age. And marriage is the stage in life where one begins to bring new lives into the world, adding life to the world.
Back to the Bat Mitzvah gift. The exact amount would depend on personal circumstances, such as how much you can afford and how close you are to the Bat Mitzvah young lady, and also should take into account local custom.
Money is not the only type of gift that is appropriate. Anything you feel that that would be appropriate would make a lovely gift as well. If you are not sure what she would like, perhaps ask her parents or her — although if you ask her, she may mention an item beyond your budget and will probably be disappointed if she doesn’t receive it!
And although the custom with money is 18 or multiples of 18, this is only a custom and any other amount you feel appropriate is also fine and I am certain will be accepted with gratitude and the joyous spirit of the occasion.
*It is interesting to note that the central daily prayer is called “Shmoneh Esreh”, which means “18”. The prayer consists of a series of blessings in which we ask for the essentials in our everyday life, such as wisdom, good health and financial success.
