How Much money for a girls bat mitzvah?

The reason someone suggested $18.00 is because is the Jewish religion, there are no "real" numbers, the letters are also the numbers. The letters that make the number 18 also translates to mean "life", pronounced "chai" in Hebrew. If you wish, give $36.00, and write in the card "double chai". Any multiple of the number 18 is perfectly reasonable and acceptable. I hope you are going to the Bat Mitzvah--you should have a great time! You are very thoughtful!