Multiples of $18 is the tradition.
You'll need to ask.
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.
That depends on how much money you want to spend.
nothing if you don't give them something
Not much
as much as you would in regular monopoly
They would not buy a PS1 from you.
$199.99
They do not purchase a PS1
Even though a Bat Mitzvah is a very religious ceremony for the turning age of 12 or 13,You can buy them a present you would buy a Non-Jewish person for their birthday.(Money,Toys,Electronics,Clothes,Etc.)Do not bring it to the ceremony though,Just the party.
Simply give as much as you feel comfortable with. If many family members are going, try seeing how much each person can contribute and then give everything that you have come up with.
Around $30