The amount of money to give for a bat mitzvah is a personal decision with no strict guidelines nor any single, authoritative answer. Based on Jewish tradition, some prefer to give multiples of $18 because the Hebrew numerals for 18 mean life, but this is neither mandatory nor universal.
Giving money is convenient and lets the recipient use it as she wishes. This flexibility allows the gift to be used for purposes, such as an education fund or to purchase something desired. As well as cash or check, the giver may also consider getting a gift card in a reasonable amount. Again, the issue of what is reasonable can only be decided by the giver. If all else fails, the giver can ask the parents whether they feel a proposed amount is satisfactory.