The reason $18.00, or a multiple of 18 is so often given is because in Judaism, 18 is a very significant number.
18 means "Chai", in Hebrew, which in English, means "Life".
Hope this helps!
Clarification:
In Hebrew, each letter of the alphabet has a numerical value. The letters that make up the word 'chai', which means life, have the value of 18. 18 doesn't actually mean 'chai'.
There's no religious significance to the gift-giving. However, it is a matter of courtesy, common practice, respect and friendship.
Any gift that you would give to a boy or girl is a suitable bar mitzvah gift. Just because it is a bar mitzvah doesn't mean it should be treated specially. Another popular bar mitzvah gift is money in multiples of 18.
A gift card from his favorite store or restaurant.
A good Bat mitzvah gift of money really depends on how close you are to this person. The Jewish tradition is to give gifts of money in multiples of 18.
Greeting cards, checks, or gift cards.
Greeting cards, checks, or gift cards.
Thank you for your generous gift on the occasion of my Bar Mitvah.
Congratulate her on her Bat mitzvah and tell her you know she did a good job on the bema. Tell her you have been thinking of her but can't make it to the bat mitzvah and that you are sending a small gift. You could find a picture of the two of you and frame it or give her a nice necklace to remind her of your friendship and her bat mitzvah.
Obviously, it is a difficult thing to tell someone how much money they should give as an appropriate gift for a Bar Mitzvah, since everyone's financial situation is very different and every Bar Mitzvah participant expects different levels of gratitude from different people. However, $36 is perfectly appropriate as a gift to give your classmate for his Bar Mitzvah, even though it would probably be an inappropriate amount for a close, working relative.
giving someone as a gift or wering in necks
You can't go wrong with money. The word life in Hebrew CHAI has the numerical value of 18, therefore people often give bar mitzvah money gifts in multiples of 18 ($18, $36 etc).If you were just friends with the boy who is even having the bar mitzvah, you could just give him something like an iTunes gift card or something like that. Same thing with a girl who is having a bat mitzvah.Important:If the synagogue is Conservative or Orthodox, never try to give a monetary gift (this include gift cards as they're the equivalent of cash) at a bar mitzvah. The reason is that Jews do not handle money on Shabbat (the sabbath). In fact, it's not a good idea to bring a gift of any kind to the synagogue. Either drop off the gift at the family's home in advance or wait until the party.
Something that can represent adulthood and responsibility: a book, tickets to a live concert/show, a trip, money, etc.