It is customary at bat mitzvahs to give cash gifts in $18 increments. This practice celebrates the sacred Hebrew word and symbol "chai," which means "life" in the Jewish faith.
According to About.com, the gematria in Judaism is an ancient tradition that assigns each Hebrew letter with a numerical value. Using this Jewish classification system, the two Hebrew letters "het," and "yud," collectively add up to the number 18, as het is assigned a value of 8 and yud is assigned a value of 10.
Therefore, giving a cash bat mitzvah gift of $18, $36, $54, $72 or another monetary amount based upon this $18 dollar increment system literally symbolizes the gift of "life" to the recipient.
According to the Bar & Bat Mitzah Guide, the standard bat mitzah guide is near the $50 range, so $54 would be the ideal gift. However, it is important to note that while members of the Jewish faith consider a gift of this sort as extremely welcome and special, individuals should not feel pressured to give bat mitzah presents beyond their means. Any cash gift to the bat mitzah recipient is seen as a touching, friendly gesture even if it is not align with this customary, $18 dollar increment tradition.