How much money to spend on a Bar or Bat Mitzvah gift?

There are several factors to consider when deciding how much money to give or to spend on a Bar/Bat-Mitzvah gift:

How close are you to the Bar/Bat-Mitzvah boy/girl – the closer you are to the Bar/Bat-Mitzvah boy/girl, the more you should give.
How many people are joining you to the party – you should try to at [...]