How much money should I give for Bat Mitzvah?

Here is the background of how closely i know the family: I came from Poland as an au pair. I lived with that family for a year and babysat the boy. It was 11 years ago. The family since has moved to a different state, but I see them every year when they come back to visit their family. I am not going to be able to attend. I am also married, so the gift technically would be coming from me and my husband. I know that a popular present at such occasions is a monetary gift in multiples of 18 which is the number of life in Judaism. Would 54 be ok or should i go as high as 90 or 108?